I am an INTJ and my favourite type of mbti is INFJ. I was wondering If I could have some great NT's pick apart just why an INTJ would be so attracted to INFJ's. Two of my best friends are very intelligent INFJ's. I have noticed they are very moral, while I, understanding morality as a relative viewpoint, am not.
You really don't know?
Well, for one thing... INFJs and INTJs are very close to being the same type. We've both got dominant Ni, and inferior Se. Meaning we share a lot of the same interests, and a lot of the same problem areas and discomforts.
We both tend to be fairly balanced on T/F, and the main difference is that INTJs have Te and Fi, while INFJs have Fe and Ti. This provides just enough difference to make things interesting, while there's still a LOT of similarity.
Also, one small bone to pick... being moral doesn't imply a failure to understand morality as a relative viewpoint. It simply indicates a desire to choose one perspective as a form of expression. That would be like saying that choosing a single system of organization is a failure to understand that other systems can be just as effective.
Anyway, this is the theoretical reason. I'm not quite as clever as an NT, but I can tell you that even they, as clever as they are, are going to need more information if you want them to tell you the reason that you personally (and not just as an INTJ) like INFJs a lot.
Oh, I almost forgot to add... INTJs are some of my favorite people as well. They're very imaginative and good at execution. After having engaged NTPs, I've found them a bit wanting intellectually, but they make up for this with their creativity, perceptiveness, passion, determination/willpower, and practical abilities. INTPs make better logicians, but INTJs make better friends and are more effective at achieving their goals.
In summary, I'd say INTPs are better at logic, while INTJs are better at everything else, including mastery of a particular subject.
Note that these are all rather gross generalizations and may not apply to individual INTs.