Heavy Metal: See also Every other form of music that was started as rebellion but has been sapped by popular culture into garbage see alsoGodSmack.
Metal is great when its done right, and there are a lot of bands out there doing it right, you just don't ever hear about them. I don't really listen to it anymore, but I "get" it, and its fun while it lasts.
They yell because they are pissed (whether or not its justifiable anger.). The music is fast, because they pissed (personally I think punks do it better, word.)
If you can get passed that though, you will find many of the worlds most talented musicians playing metal music, its complex as fuck sometimes, and you have to respect that, even if it doesn't sound all to pleasing.