basically you have a lot of traits that are great when working together to deal with something other then themselves, like a task at hand, an external challenge, a mutual project, etc'. but the very same traits create problem when dealing with each other and leads to a lot of difficulties in fostering a good dynamic.
in the second post i added the disclaimer, that me and many others here - INFJs and ENTPs - are likely to be very biased against that idea for the wrong (or rather irrational) reasons:
basically the MBTI has created a set of hypothetical "couples" that supposedly work well together:
NTPs and NFJs
NFPs and NTJs
STPs and SFJs
SFPs and STJs
the basis for it is not without merit. it includes judgement functions in opposite order (Ti>Fe & Fe>Ti, Fi>Te & Te>Fi), and apposing perceiving functions (Ne & Ni, Se & Si). the first part means you have a similar way of thinking but with different strengths. the second part means that you are able to gauge different information from the subject matter at hand. those really are very positive interactions.
but it also creates the reverse affect: just like you have people who go with the MBTI's suggestion and idealize the idea of a relationship with the other type, you also have people who rebel against the idea of it. in both cases the reason can include personal experience, but in the later its a negative one. in other words you have people who had relationships with their "ideal match" that ended, caused pain, and they associate it with that type as a whole (i'm somewhat of an extreme example guilty of doing that myself).