How is a mindstream associated with human forms?

I think this question is distinct from those asking what is reincarnated . I understand the distinction between the mindstream and a soul, and the *simile* of passing on a flame. I also understand that an association is made between a mindstream and human form, most often in the womb. What is it that makes and maintains this mental-physical association endure throughout a human lifetime? Why is there a tendency for the association to result in a first-person perspective/sense of self?