What makes a good portrait a good portrait is not the amount of collaboration, it is a photographer’s willingness to take what s/he wants. If that sounds too Nietzsche to you, then, well, maybe you don’t want to take portraits.
It’s much, much harder to be selfish when working with someone after having gained consent. This involves asking (and, possibly, rejection). It gets hardest when there’s more at stake than just a good picture – let’s face it, even the greatest photographs are just photographs, really not more, but also most certainly not less.