I think there are some hints in this interview with <PERSON>: http://www.avclub.com/article/milos-forman-13764
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 history in which every song is a gem.
But here he's talking about another film he directed, Taking Off. Notice what he says about hippies in their environment:
I considered a portrait of these young kids, called hippies in those times. But what I discovered... I spent a lot of time in the Lower East Side, seeing these crash pads, and I found them very, very boring. On the other hand, when I talked to the parents of runaway kids, I found them much more interesting, because they were in panic. 899
They were doing things, they were trying to find their children, they were trying to understand their children. The children just spent their time lying in the crash pad, smoking pot and looking at the ceiling. It was very boring. That's why I turned the focus of Taking Off on the parents.
So now it's time to direct <PERSON>; is it likely he's going to retell the story he saw on stage, when he thinks (overall) their lifestyle is boring?
I think instead he looked for ways to introduce conflict, and that started (with a classic Man vs Man) with <PERSON> meeting <PERSON>. And it's a good setup: a naive, fish out of water, law-abiding, soon to be soldier meets a gregarious, law-breaking, anti-war rebel. <PERSON> <PERSON>, our protagonist, thought he knew is being challenged.
I agree with what you said about <PERSON>'s infatuation with <PERSON>, but some love interest was necessary in order to provide a reason for the trip to <PERSON>'s base. And also to provide the means to get <PERSON> shipped off to war. After all, it was <PERSON> being just a bit too clever and he lost this one.
The really big theme, for me, are the changes designed for the three main characters:
* <PERSON> goes from naive to getting having his eyes opened
* <PERSON> leaves the authoritarian high society and is free
* <PERSON> for the first time in his life has to follow rules
Despite <PERSON> saying, above, "I just liked the music!", I think he made the changes he felt he needed to make to keep the story interesting. 627