Highlander, Uulau, Starry, Southern Kross, and Glycerine help me understand and help me not feel so frustrated, and Rasofy and ShortnSweet help by making me laugh. Dunno how they feel about me, but I grow in affection towards them. Toward you, not so much. But life is long and who knows what may happen in the future.
One thing to remember, Tilty, is that while both INFPs and INFJs are generally very loving, caring people, they're "differently prickly." I see this different prickliness as mostly being an Ni/Si thing, though Fi and Fe play a bit of a role, too.
INFPs are prickly in terms of tone, in terms of overall gestalt. There will be things that simply rub them the wrong way, and there is no obvious cause and effect. The feeling
just is, and the notion of a proximate cause seems nonsensical. (There were lots of causes, not just one. It's everything at once, and there is no neat and tidy story to explain it.) [MENTION=5999]PeaceBaby[/MENTION] is essentially saying, "That rubs me the wrong way. Can you see why?"
But she never quite explains explicitly why. It's just like what annoys me from INTPs: they'll hint, cajole, and prod that you're wrong about something, but never explicitly say why it's wrong. It's always "an exercise left for the reader." You either figure it out for yourself, or you don't. Similarly, the essence of the INFP's reaction is always left for others to figure out, based on clues that don't quite make sense. A large part of the problem is that the INxP version of "why" is answered in terms of Si: it is simply true, it exists because all of this other stuff exists, and if it weren't true, then the entire world wouldn't make sense. (This is why I used the word "gestalt" earlier.)
In my case, I understand INFPs, because I can figure out how to resonate with that Fi point of view, but I still have to translate it into my own sense of Ni cause-and-effect. With INTPs, if they haven't bothered trying to use more cause-and-effect language, I end up having to ping them over and over again, gradually getting bits and pieces of data, until I finally get the piece that lets my Ni organize it all into its own Ni system. I suspect this annoys the INTPs greatly, and if they're not already a friend, they're wondering how I could be so clueless, because isn't the truth so very obvious?
INFJs, on the other hand, are prickly about causes and responsibilities. They need to know
why you did what you did, why you feel what you feel. There had to be a
reason, and they can find and understand that reason, they're for the most part happy. Without that reason, they start probing with questions that sound remarkably accusatory (INTJs do the same thing on more fact-based matters). An INTJ might say, "Are you done, yet?" and unintentionally convey impatience, an accusation that one SHOULD be done by now, when it really is just a factual query (the INTJ needs to know whether it's OK to start, yet). Similarly, an INFJ might ask, "Why did you do that?" and actually be wondering why in a non-accusatory way, but it will come across as completely accusatory and sound like they're implying that of course no one would ever do that, that there isn't possibly a good reason for doing that.
I end up being able to understand INFJs, for the most part, because I always have a response to "Why did you do that?" There is usually a
reason behind what I'm doing, though it's in Te terms, not Fe terms. The only real issue I run into with INFJs is that they'll occasionally accuse me of doing something for an Fe-style reason which never would have occurred to me (e.g., thinking that it's because I like or hate someone, not seeing that my true reason is that it optimized things in a practical way).
Thus we set the stage for the battle: the INFPs rely on the
tone of the INFJ's means of addressing uncertainty, even as the INFPs seem to stonewall the INFJ's legitimate questions. The INFP's responses in turn tend to suggest that the INFJ is doing something wrong, that the INFJ doesn't really care (or isn't really emotionally aware), when really it's just the INFP is puzzled at the apparent lack of understanding.
This is why I believe it's mostly Si/Ni issues: INFPs and INFJs intellectually inhabit different "problem spaces", and translating between them is difficult. If my experience with INTPs is any guide, it's more on YOU (INFP or INFJ) to figure out how to communicate across that barrier than it's on the other person. If you're an INFJ having issues with an INFP, try saying, "I'm trying to understand. Could you explain more to help me understand?" instead of "Why did you do that?" The former will resonate as cooperative with the INFP, while the latter will come across as accusatory. Moreover, be especially careful about offering up hypotheses about why the INFP does or feels anything: if the hypothesis isn't in the form of a question with lots of "maybes" and "perhaps" making it clear that it is a question, not an accusation, it will come across as an accusation. Being mocking or sarcastic is generally out of the question, with INFPs, unless you're on
very good terms. If you're an INFP having issues with an INFJ, try stating your feelings in a more declarative, "if-then" kind of way, e.g., "When thus-and-such happens, it makes me feel bad, and I tend to react by doing X," instead of, "Isn't it obvious why I'm reacting the way I am?" Focus on specific cases: instead of discussing an overall principle, present an application of the principle. The tough part on the INFP side is shrugging off everything that sounds accusatory: if you get too upset, it becomes difficult to focus on the communication.
I've found that it's generally very easy to get along with other people, as long as you give them the benefit of the doubt, and understand that their version of prickliness isn't wrong or evil, it's just how they're wired. It's fairly easy to work around if you see it, and especially easy to work around if both sides see it.

Only the most ornery/crabby people don't respond well to this approach, at which point it doesn't matter what type you are ... they don't get along with most people.