Because ENFPs are often highly intellectual (sort of), they tend to have extended introverted phases earlier in life. They can be that boy or girl who reads a book at lunch. They tend to have a great turn of phrase, and tend to appreciate literature. Reading books is a rather solitary activity, so it lends itself to introverting.
I think they tend to explore, develop and ascertain their Fi values at this time. They often develop a sort of shell (very FiSi-influenced [Ne, too, though]) As they get older, they tend to come out of this shell. As they do, they tend to display NeFi and/or NeTe. The more introverted they are, the later they tend to come out of this shell. Strong-extrovert ENFPs might never go through these introverted phases. Or they might be less frequent / shorter-lived.
I think they tend to be more introverted than other extroverts for these reasons, and because Fi is very sensitive, and felt very deeply, and so along with intellectual Ne, lends itself well to introversion.
Because of the imagination and subjectivity of it all (N and F), their cognition allows them to create a whole world largely of their own imagination, which might be burst if it comes into too much contact with external objective reality (Te & Se) (hence why more immature ENFPs often reject Te and Se, in an infantile attempt to preserve their created reality, sans interference from objective reality).
Would be nice to finally meet one.
ok but seriously, we don't want your eyeballs to fall into your insides, it's just going to be all brown and stuff.
