I think we can find an answer in Forman's biography and filmography.
For me, Forman is about one thing (as we can see in Hair and One Flew Over the Cuckoo's Nest), the conflict between an individual and society that claims adjusting and obedience. In Hair it's Claude against the U.S. Army, in One Flew Over the Cuckoo's Nest it's Mac against the mental ...
I think there are some hints in this interview with Forman.
First, I found this amusing:
O: What about Hair? Did something particular draw you to the play?
MF: Songs. I saw the very first public preview on Broadway of Hair,
when I was here in 1967 for some sort of festival or something. I just
loved every song. It's one of the three musicals in ...