They will - you don't understand that till you're older. I read this great book called "Play" that explains how fighting establishes your instincts as an adult - it teaches you how to judge when you've gone too far, and the difference between joking and hurting, etc. It's really fascinating. They're just learning how to be civilized adults by being uncivilized kids.
Don't be sad.
Here's the book: http://amzn.to/tOLwIb