I dated an INFJ years ago - the only guy I've ever been in love with. I loved that we didn't have to talk in order to be on the same page, like scary telepathy. But, he was moody - actually, every NFJ I know is moody - and his feelings, though internalized, really seemed to take him over, like he just couldn't handle them sometimes. It was driving my Fi up the wall. I have to say though, the chemistry was insane.
That's how it is with my INFP (and yes, the chemistry is DEFINITELY amazing

), but minus the moodiness on my part, cause I'm very level, even in stressful circumstances/touchy situations and such. I honestly think most any other INFJ guy who has had all the stuff built up in their life over time that I have, who is also highly sensitive, would have an awfully hard time having a relationship with my INFP--at least at my age, anyways--because if I wasn't nearly as stable I am, well....there are all kinds of things that could be causing difficulties, or even kill the relationship entirely, hurting both people a lot in the process, but I can see through all that quite easily to the root of everything like any emotional outbursts she has or misunderstandings that could be taken personally, among other things.
I'm able to take care of that stuff in a way that is very firm and uncompromising as to what needs to be done, but at the same time never hurts her, and does not push her away, but rather, just gives her a little shove in the right direction, so to speak, and helping her back up if she stumbles. She's said that I'm always totally right when I do that (We could probably end up talking for hours when I get into it. It makes for very good conversational material that helps bring us closer, which I always love.

), but that it's just really really hard for her; I know that she'll get better with those things in time, though it could very well take quite a while, as she also has various emotional disorders, depression, and even PTSD (most likely from her father abusing her, though she no longer has to worry about that because he was finally dealt with just this past week), and her ways of coping with those things are very much ingrained, and I'm working with her on changing them to better methods that don't just hurt herself further.
Most of that is largely intuitive for me, though, as I seem to just naturally understand her a lot better than anyone else she knows. I am very good at understanding people in general, but with the people I can really connect with (which are few and far between), it's on an entirely different level, like to the point of frequent non-verbal understanding of exactly everything the other person is feeling, and pretty often I even end up personally feeling myself what they're feeling without having any actual knowledge of it....which can get a bit overwhelming at times (though never out of control), but I'm very glad for it because understanding her is very important to me and that does help a lot.
I think the biggest problem for an INFJ/INFP relationship would be one or both people taking these issues (This could, of course, be an issue of varying degrees in many different relationships, but it can easily be compounded and greatly magnified in this specific type of relationship.) and trying to solve them on merely the surface level, instead of actually taking the time to see exactly WHY those reactions/perceptions/etc. happen by going backward through their thought processes each step to find the base cause and take care of that before anything else. I know a lot of times this can be initially VERY difficult and painful to do because those things are very deeply seated in their psyche, and if one or both people can't handle the pain of it, it could very well end up making the relationship impossible to be healthy, or even exist at all, but it's very much worth the effort, because taking care of all the superficial stuff is merely a temporary fix.
One other thing I should mention is that I think an INFP/INFJ relationship would work out a lot better (this is just based on the majority of both of them being Enneagram type 4 [which seems to be more common] or 9) if one is 4 and the other is 9 (Healthy 9s are a very stabilizing factor in any sort of relationship.). If they were both the same type--especially if they're both 4s--the difficulties and insecurities that each person has would reflect off each other getting worse each time, while also creating a dynamic that is unchallenging and stagnant, but if it consisted of one of each, it would be highly complimentary. Here's what The Enneagram Institute says about that, which I think is perfectly descriptive of my relationship...except for the part about reading papers. lol
What Each Type Brings to the Relationship
This can be, paradoxically, both a very comfortable—and yet exciting—relationship pair. Enneagram Fours and Nines are both withdrawn and private, sensitive to the feelings and needs of the other, and empathetic to the suffering of others. Both can be tender-hearted and highly sympathetic to the suffering that they find in the world and in each other. Both want to find a deep connection with the other, and yet, both also want a certain degree of autonomy and insist on a very real degree of privacy. Both Fours and Nines can be highly creative, and as a pair they enthusiastically support the other's creativity and give the other a good deal of space in which to develop their talents. Both are idealistic and want to connect deeply with someone, feeling that they are on a search for their soul mate, the one person in the world with whom they can completely connect and be themselves.
Both Fours and Nines also bring a sensuality and love of comfort that is noteworthy; this may express itself in their lifestyle, traveling habits, and in their sexual and other intimate activities. This is a couple that likes to stay in bed all Sunday morning, reading the papers and talking. Each brings passion and an appreciation of the other coupled with a desire to be comfortable and build a life with the other. Fours can make Nines become more intense and expressive about how they feel, while Nines can allow Fours to feel understood and accepted for who they are. Fours are good at naming feelings and pinpointing emotional states; Nines are good at creating an atmosphere of nonjudgmental acceptance, Nines may even enjoy the emotional storms and dramas that Fours occasionally get into, feeling that it adds spice to their life together. A lot of the pleasure and passion of this couple is nonverbal in the depth of the understanding that each has for the other.
This part, however, thankfully isn't a problem for us at all.
Potential Trouble Spots or Issues
The biggest area of conflict between Fours and Nines is that each tends to react differently as stress increases: Fours become more emotionally volatile and demanding, while Nines become more disengaged and impossible to get through to. Fours can feel too unstable and dramatic, unpredictable and moody for Nines, while Nines can feel too unresponsive and emotionally inert, unsatisfying and uncommunicative for Fours. Nines can become angered by the Four's sense of entitlement and demands for attention and exemption. Fours can become angered by Nines' irresponsibility and apparent inability to learn from their mistakes and experiences. If conflicts and tensions increase between them, Nines can shut down more and more so that communication stops, and they give Fours the subtle message that they don't want to hear their reactions or deal with their feelings. Fours can feel that talking with Nines is like playing tennis with yourself—there's no one to hit the ball back—and there is too little relating in the relationship. Contempt for what Fours see is being boring and ineffectual can end the relationship.
Both types look for partners who seem to embody some qualities that they feel they do not have themselves: Nines seek strong, high energy partners, whereas Fours seek partners who possess some ego-ideal that they feel they are missing themselves. In lower functioning Fours and Nines, neither energizes the other and both get stuck in a morass of inhibited anger, resentment, and irritation with each other. Fours tend to give up on the Nines (as being hopelessly inert and dull), whereas Nines tend to give up on Fours (as being too demanding and emotionally volatile).