First off I need to add a caveat, I'm a single ENTP so maybe my intentions are to try and swoon one of you INFJs but what's wrong with that.
No really (well, additionally), I wrote this to get these thoughts out of my head and thought I would share because there does seem to be some confusion about how these functions work and just what it is to be an introvert or extrovert.
I will say there has to be a huge level of self awareness on the ENTP side, something I think is rare before 30 and I wouldn't trust a young ENTP myself so a word of caution to any heartbreak that may arise from pursuits of young ENTPs.
To fully explain this I need to go more in depth on MBTI itself so I'm sorry in advance if you already know this as well as for my inability to be terse. MBTI is not just 8 letters and being an INFJ for example doesn't mean only what the letters say at face value. As an example an ENFJ is not just the extroverted version of the INFJ, they are VERY different. The two middle letters are the main focus each with an extroverted or introverted possibility. So you can have Ne, Ni, Te, Ti, Fe, Fi, Se and Si in many different orders. To give an example, Se is very sensory based, an ESFP would lead with this function as their main way (or least energy) of processing the world and making choices. To stereo type, the ESFP is the classic sorority girl who will date a douche bag asshole based only on his looks. In the case of the ESFP this Se function is followed up with Fi which is about internal feelings, when used correctly this translates to a self aware person, when wrong very selfish. Also the reason the ESFP is all about "me" and what is everybody going to do for "me". What a messed up function (Se) to lead with right!
Now if you move on to a person with a dominant function of Ti such as with an INTP, you have the classic perfectionist engineer with tape on their glasses who doesn't even notice how messed up their hair is. Mainly because they lead Ti, Ne, Si and Fe leaving them with sensortardation (you and I have this as well) because we don't notice surroundings and do stupid things like walk into closed doors or forget we just brushed our teeth and brush them again, this is due to how in our own heads we are, always thinking. So again the INTP totally lacking the Se function would be oblivious to their appearance where as the ESFP would care only about their appearance.
With that said, the INFJ is Ni, Fe, Ti and lastly Se leaving them with a dominant (or no energy to use) function of Ni that generates endless possibilities in an internal world. This is the reason they can picture things clearly and in such detail when they read (almost as if they're watching a movie in their head), this is why they can be alone in their own mind for long amounts of time and never get bored. With a secondary function of Fe, this is what allows them such an amazing ability to read people and is also what makes them such an enigma, they generate all these possibilities yet follow them up with Fe, a function that's VERY people based. They feel SO amazingly deep because they can visualize what other people feel in an endless world with no boundaries or rules that they haven't given it.
To move on the ENTP is Ne, Ti, Fe and Si leaving them a dominant function of Ne, so they're as vivid as the INFJ except in the external world, their fantasies are about building things and with any and every thing they look at they wonder how it works and if they can make it better. They follow this up with Ti so they take all these possibilities and filter them with internal thinking. They can look at something very simple, imagine it 500 other ways and quickly think about what would work the best. It also affords them the luxury of being extremely quick witted and good at debates, because they can generate 50 possibilities and figure out what works best before most people can think of one. So if INFJs are the enigma having the Ni function followed by Fe (one being so internal the other being so people driven) ENTPs would be the paradox in that their first two functions are not people based and even though their dominant function is an extroverted one it's the extroverted function that doesn't need other people, leaving them the most introverted of all the extroverts. In fact with INFJs secondary function of Fe they're more extroverted than an ENTP although it doesn't look that way on the surface. Picture the INFJ as water dripping into a bucket, picture the ENTP when the bucket finally tips.
The ENTP/INFJ dynamic is so amazing because if you look at how we look at the world it's:
Ni Fe Ti Se
Ne Ti Fe Si
What that boils down to is picture a sock and somebody reaching in and pulling it inside out. ENTPs have the same depth INFJs do in a different way, ENTPs are the same as INFJs yet opposed in a way that is more reversed than opposite. The functions line up in a way that leaves both feeling totally understood yet with endless learning possibilities and total balance. They're like mental playgrounds for one another and it never gets old because both worlds are based in endless possibilities. This same dynamic holds true for the ENFP/INTJ.
Where the four dominant intuitives differer so much from other types (and why this dynamic is so interesting among the ENTP/INFJ) is our satisfaction (or mostly, lack of) in relationships due to our desire for a deep mental connection, both are looking for mind mates and both want the ideal relationship. Overall we (dominant intuitives) have the most consistent moral systems and are most likely to be disappointed if someone falls short of those moral standards. Both are very growth oriented, passionate, deep and unwilling to be in a relationship of routine that gets old and lazy in time. You do not "get", "capture", "confine", "own" or "cage" an ENTP, ENFP, INFJ or INTJ, those are the four types that pick you.
The issue with the difficulty in starting an ENTP/INFJ relationship isn't limited to one thing. First off, neither is used to being understood so there's this never ending clarification and "know what I'm saying", "if that makes sense" issue. Both are in dismay somebody else understands with such ease. However because we take different paths to the same place we'll ask things like "why'd you do that" just to understand, when explained the other person totally gets it but because we're so used to the same thing happening with a different outcome it preys on our frustration or issues of never feeling understood. Secondly the INFJ is hard to get out of their shell and the ENTP is quick to run back to theirs. The INFJ has this feeling initially that they'll never get the space they want because they see the ENTP extroverting and the ENTP swears you're not interested. The INFJ will have this constant push/pull I want it no I don't issue and have to fight to get the flood gate open. The ENTP will have this run while you still can issue and have a tough time keeping the flood gate from shutting. Both speak truly and honestly but have a tendency to try reading between the lines with the other when there's nothing there to be read. We apply our own motivations to the other persons actions which is never correct (different path to the same place) and this all takes some excellent and very open communication. These issues don't last long however so at least that's a plus. We're both a type that never really lets people in and we're both amazing at reading people so we're faced with an issue of holy shit this is going to hurt if it doesn't work so both need to want love more than the fear of having it. We're both so whole alone but together there's an overlapping of abilities that gives the other a peephole (I read that four times and kept thinking there was a missing p) into a whole different world. I can tell you right off the bat, if I (as an ENTP) had internal issues I was hiding I wouldn't even consider pursuing an INFJ until I had my head on right because I know it would be picked up on instantly.
There two things I would like to add while I'm thinking about them. I've compared the ENTP and INFJ to opposite sides of the same coin saying we're exactly the same yet totally different.
I would say the ENTP and INFJ are both the oddity of the MBTI world on the introverted and extroverted side. We don't really fit in anyplace and are the two types that most often feel quirky, misunderstood and like there is no way anybody will every accept much less understand and love how different we are.
Anyway, what dawned on me is the false low score we often get on our E function. My dominant Ne function is very high yet I will sometimes only score 54% E and on occasion I will test as INTP. This is because depending on the test I will answer all questions like "you feel comfortable in a crowd" with a yes (standard extrovert answer) yet also answer all questions like "you require a lot of alone time" with a yes, standard introvert answer. This conflict is due to leading Ne, an extroverted function that doesn't need people at all.
By contrast and backing up everything else I've said. INFJs often falsely test as INFPs, because while the ENTP is the most introverted extrovert the INFJ is the least orderly J. With MBTI these are the only two anomalies like this I'm aware of.
I have never probed enough on this topic to explain why this is the case with INFJs and that J function but I do find it interesting and in line with everything else I've said.
Also, ENTPs are the slowest to mature and the INFJ is of course the quickest so I think a 5 to 10 year age difference with the ENTP being older is ideal if not essential. The downside for the INFJ maturing so quickly is because you already feel so different and alone (the ENTP does as well but being the extrovert with an over sized ego they haven't noticed it yet and is enjoying still thinking they're better than everybody else) you also don't feel like you fit in with other people your age so by the time the ENTP has matured the INFJ is feeling jaded, frustrated and starting to let go of that ideal picture of love they've always had and can find in the ENTP.
Damn this was long, sorry, only a saint or INFJ (same thing I guess less the dark humor) would read all of this.