Great post, and I see the dynamic also. I also think I know why it happens. It seems like coach has a real need to want to make something out of nothing. Sometimes it's because of a challenging personality that he wants to transform, and sometimes it's an undervalued athlete that he wants to try to turn into a superstar. Coach gets a real charge out of this facet of his job.
I think he learned his lesson with the challenging personalities, because he seems to be truly shying away from them after being burned one too many times.