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?
What is it that makes and maintains this mental-physical association endure throughout a human lifetime?


It's an ongoing conditioned process whereby a state conditions the next.
Imagine a line of domino's. You tip one over, that one tips the next over and so on. There are the domino's; and there are different (invisible) forces playing a part in how the next domino is tipped. 
(It's not a great comparison though. Cause the domino's in the line are already there when in actuality that's not the case. But I haven't found a better comparison yet, the candle simile doesn't work for me.)

So, a state of matter-mind-combination ceases and makes room for the appearing of the next matter-mind-combination (at least in the sense realm). 
The ceasing of the mind-matter-combination is a condition for the arising of the next. But so are different other conditioning forces. 

The Patthana (last part of the Abhidhamma) explains 24 different conditioning forces that play a role in the arising of the next states and the maintaining of the process. Important is also to understand that there are always several conditioning forces active, not just one.
You might want to read this book. Or just the chapter of the Patthana in the compendium of the Abhidhamma, by Bhikkhu Bodhi.

By the way, at a certain point it's possible to see the conditioning of this ongoing arising and ceasing in meditation. It's fascinating, at least for me.


  Why is there a tendency for the association to result in a first-person perspective/sense of self?


That's due to conditioning. Delusion (which is there when one experiences this first-person sense of self) is basically embedded in every newly arising matter-mind-combo. It has to be broken down, replaced by new conditioning. 

The practice of what's usually called vipassana meditation (or meditation based on the Satipatthana sutta) makes this possible. From own experience I can say that with vipassana the experience of first person becomes the experience of being the observer (2nd person) of the ongoing arising and ceasing of the matter-mind-combo. And later the experience shifts even further away to more and more neutral, so the feeling of being the observer also drops away. By then there is no more experience of liking, disliking; no identification with what ever khanda, no kilesa and so on. (Upekkha nana).

So, it's a matter of replacing old conditioning with new one. That can take time though. Depending, again, on the way your mind stream is conditioned and how quickly your mind can let go of old ideas and so on.

Hope this helps a bit.