Several reasons, from the therapeutic to the evolutionary. I expect my answer to not be particularly popular since it requires some serious introspection into both gender's psycho-pathologies, also it's a bit long, but such a question is in fact quite complex. Ideals vs Reality - Let's get this out of the way first.
Paula knows firsthand about the “expulsive power of a new affection.” Take the quiz in the first chapter and see if this book is for you or for someone you know.
Some NTs can't stand not having friends over or being alone, which is the same sort of neediness.