It's pretty simple. The Ni, Fe, Ti, and Se combo makes for a very exclusive club. No one else gets us, nor can they meet our standards.
NFJs are best paired with other NFJs because we have the same expectations and quirks. Fe makes us really difficult to deal with but also makes us have a hard time appreciating anyone who doesn't meet the standards that we set for ourselves and others. Ni makes us very much intuitive and direct all at once. We're best paired with our own kind.
STPs make a very good second pick for NFJs because we're the same, just on a broader level. We share Ni, Fe, Ti, and Se. ISTPs are better for us than ESTPs because they share our need for tranquility, but ESTPs may be a better match for a female INFJ. While they're more down to earth and live in the moment more, STPs share our direct approach to life and we can admire each other because our aspirations are in one another.
Other types can do very well outside their 'box', and in fact some types require it. Types that have Ne can greatly appreciate those who are different than themselves. Types that have Fi can pair with any other type who doesn't expect anything from them. This means that the best type for an ENTP is an INTP or an INFP. The best type for an ENFP is an INFP or an INTJ. I'm not going to bother getting into the S types, other than to say that they all seem to do pretty well with each other except for STJs and SFPs who can clash quite a bit... but then STJs clash with pretty much everyone, even themsleves. It's what they do.
Those that are not NFJs or STPs may get along well with us for some time, and are usually well served by us, but eventually we will grow tired of the conflict and clashes and want to leave. It'll take us quite a while to get out of one of these relationships because we don't want to hurt the people we're in them with. So, be warned. If you get into a relationship with one of these, it is likely to turn sour, and may leave you in an emotional quagmire for far longer than you intended to be.