We were going to say that Oronte and that other guy who had lawsuits against Alceste were the antagonists, but the thing is, he had problems long before they came along. Alceste hates people, criticizes them, and is an all-around curmudgeon of the "get-off-my-lawn" type.
Why is he like that? Well look no further than your title, Shmooperinos. He's a misanthrope. He hates humanity. That's what is stopping him from having the totally sweet life that he has always wanted. Of course, everyone else knows that he hates them so they hate him back. It's a vicious circle.
You could say that Alceste is his own worst enemy, since most of his problems could be solved by not being so angry, but he'd probably just say that you're a hypocrite.
It's OK. We think you're probably right.