INTPs often tend to fall for INFJs, particularly in a heterosexual relationship where the male is INTP and the female is INFJ. Since I am in this one, I can give my theory on why these types fall for each other:
Why INFJs Fall for INTPs:
1. INTPs are honest. In fact, they may be the most straight-forward and truthful of the types...to the point that their precision with the truth can seem pedantic to others, and even socially awkward when they are attempting to see social rituals through their objective lense. Still, INFJs really love this quality, and often come to rely on the INTP's sense of honesty and truth once they learn to trust the INTP.
One of the potential pitfalls of the relationship never getting off the ground is the INFJ's tendency to think they can see the motivation behind everyone's statements and actions...but an INTP often has no alternative motivation then stating what they honestly perceive to be the case. I've had INFJs and ENFJs that have gotten on my nerves by constantly questioning my objectivity because they think they can "read me" and see alternative motives when there are none there.
Much later in the relationship, when the INFJ has learned the INTP very well, this can become a huge boon though...as the few cases that the INTP has alternative motives, the INFJ can sniff that out and bring it to light, potentially resulting in enormous amounts of growth for the INTP as they purge themselves of prejudices and become even more objective (which is very healthy for INTPs).
2. INTPs make fantastic and often genius connections that impress the INFJ, and even lead her to growth for herself. While our INTJ counterparts ask the question "Does it work?" our ENTP counterparts ask, "How could it potentially be?" and ENTJs ask, "How can it be best put to use?" we INTPs ask a more fundamental and base question: "Is this true?" We ask others to tell us what it IS, not what it could be, what they think about it, how it is used, etc...just what it is.
This unique persective contributes a lot to our obsession with logical correctness and truth, and hence the attraction from #1, but it also answers vital questions an INFJ is likely to never consider, as the questions they tend to ask, such as, "Is this good for others?" and "Do people have a right to do this?" often skip over more basic questions that are answered by the INTP. Going "back" to review these questions, which is what the INTP brings to the INFJ, will often cause the INFJ to view things in a new, more precise perspective, causing much personal growth.
3. This is related to 1, but bears mentioning separately. There is very often a constant miscommunication between the types (where the INTP is being logical in explaining his theories, thinking the INFJ is very interested, while the INFJ thinks the INTP is expressing his feelings...being open and honest about the subject) that is actually beneficial to producing attraction. If there isn't a huge gap in intelligence between the types, this can be a great thing, as the INFJ can understand the theories the INTP is communicating and contribute to the conversation, even helping modify and produce new explanations for things...it makes for great conversation.
4. INTPs like sex. INFJs like sex.
Why INTPs Fall for INFJs:
1. Mystery. We INTPs love logic, mathematics, language, and anything that requires theories and explanations to learn, but we perhaps are more intrigued by those things we have extreme difficulty explaining. INFJs often have a certain "psychic" quality to them that can be extremely intriguing. They almost seem to read the thoughts of others at times. Later in the relationship, after the INFJ has learned the habits of the INTP, this quality is very beneficial, as the INFJ can "call out" the INTP on things they aren't as objective as they tend to be.
2. The INFJ mindset ("Is this good for people?" "Do people have a right to do this?") can provoke an INTP into interest in such fields as psychology, philosophy (especially ethics), sociology, and the like. INFJs, if anything, are INSPIRING to an INTP, as they provide such a different way of looking at things that the INTP is inspired to learn and make theories on subjects the INTP may not have even considered at all important before.
3. INFJs like sex. INTPs like sex.
4. Life is tough to an INTP. For the same reasons INFJs and INFPs get along well, INTPs and INFJs have a similar thing: INTPs are so counter to the dominant cultural mindset that life can be VERY wearing on us. INFJs are even called "the confidants" and this quality is very nice to an INTP. When our emotions do come out, it will only be to a select person or very very limited group of people. If an INFJ is the closest person to us in life, then this rare outpouring is made much easier for the INTP, and often makes the INFJ feel great as they are trusted with a rare phenomenon.