I used to think it was because of the foul language and how I felt verbally assaulted after I'd been to one of his shows. However, strong language in other shows didn't bother me. I'll even use vulgarity myself when the situation seems appropriate (though never profanity and I do draw a very distinct line between the two--if I say "God," it's because I'm talking to or about him).
So why is it that I don't like David Mamet plays? It's because of the characters themselves. So often they are people who exhibit the worst human qualities. They are cruel, heartless, selfish, and amoral. Many of his characters could easily be diagnosed as mentally ill--sociopaths and psychopaths.
While drama is an excellent way to explore social diseases, Mamet's outlook is far too pessimistic and ultimately lacks authenticity. The societal problem that it skirts isn't that there are people like the ones he portrays in the world. The problem is that we look at others and see monsters like the ones Mamet creates. How many times do you hear someone come out of a Mamet show and say, "I know people like that."?
I've met a lot of people in my life. While there may be people who resemble Mamet's characters and who engage in some of the behaviors, none are as lacking in empathy or soul as he portrays. When you take the time to listen, you discover that the person does have redeeming qualities. For some people, it might take a lot of listening and a lot of empathy.
Hatred is easy. It's a pretty destructive habit to have. It's far easier to scream obscenities at the person who cuts you off when you're driving than to say to yourself that perhaps that person is having a bad day or didn't see you or any of a number of reasons that would make their actions understandable. It's far easier to classify someone as an idiot, jerk, or any of a number of stronger terms that to simply acknowledge that we don't like some of their behaviors--anymore than they likely are fond of some of our own behaviors.
We get to choose how we see people. We get to choose what sort of interpretation we put on their actions. While it is not wise to be naive, it can take great courage and effort to choose to see the best in people. We could see the world through cynical eyes that believe others to be criminals, wastrels, and users. Or we could see the world through compassionate eyes that believe others to share in our own struggles and to be searching for ways to be healthy and happy.
The latter may be more difficult, but it is also far more rewarding.