Its because ENFPs live their lives romanticaly, in a novelty way, and they are drawn to the misterious, unusual people.
ENFPs usualy end up sticking around with ISTJs, INTJs, and INFJs.
Another reason would be that ENFP seek for "simple" people. Due to their impressionable nature, they are easily intrigued and even awed by people who have big influence, power and have a "superior" behavior. So they seek to relate with people who are "simple" for them, and they do find INFJs to be "simple".
I remember Mark Twain, who was a ENFP, how he used this phrase consitently in his writtings: "a simple man, surprinsingly simple man".
One note for this...ENFP never lack or find themselfs worried about "self-esteem". They are not self-confident or anything like that (althought some ENFPs, like Donald Trump is, or Caesar Millan from "The dog Whisperer"), later in life begin to have a self-confidence in them, making a very beautiful and strong impression (like a young lion, to be more poeticaly correct). They don't have a mature self-confidence, but to some of them, there can be seen something beginning there, that energy, and that practical wisdom that Aristotle talked about, coupled with courage, that makes them very beautiful.
But yes, from my experience, I have never seen ENFPs who are worried about their self-esteem. They might be feeling frustrated because they are "simple", and other people are "complicated and sophisticated", but they never fall in despair. If a "sophisticated" person, like a ESFP, comes to a "simple" person, like a ENFPs, and tries to make him feel inferior in one way or another, the ENFP will put him in his place in a "simple", but efficient fashion.