I think you answered your first question when you said, "well, although Kou doesn't love him 100%." I think for most people, knowing that they aren't loved 100% by the one they love more than anything, is painful.
yeah, i know that.
however, Ao chose to walk on that painful way.
Ao got his happy ending, that's important. :)
If you can call that a happy ending. To others its a dependent ending where he clings to an obsession like an addict refusing to give up drugs. Kou likes sleeping around. In the end he will probably continue to do it and Ao's wakeup call will probably be when Kou brings home a little gift known as stds.
In this manga, Ao got his happy ending. no matter what we said, the story won't change. in real life, there's people like Ao, whom chose to cling to what you called addiction (in this case, addiction to someone).
just so you know, my BF is that kind of person, i didn't sleep around but i played around enough to make other people called him coward and idiot. don't get the wrong idea, i'm not proud of what i did.
and about the little gift you mentioned, knowing Ao's character, perhaps he will take care of Kou the rest of his life.
Thats..not exactly a happy ending. Thats settling. Nobody grows, improves, changes, the cycle continues. Thats destructive. For me I see it as an open ending and they dont last.
it's up to you if you see it as an open ending. people has different thought. feel free to make doujinshi if you want.
i dunno why people hate the ending. i like the ending, Ao gets his love back. well, althought Kou doesnt love him 100% but at least he knows whom he has to choose. 9 years is not a short time. and i believe Kou chooses Ao not based on pity only. He loves Ao, he says that on the story (dont remember the chapter though).
but overall, i still want to punch Kou on his face! he gets my nerves! DAMN! if you confuse whom you love, dont sleep around, you damnit!