Why do people fight?

Why do people fight about stupid things? My friend and I got into a ridiculous but small fight today about baseball. We were playing in the yard and i didn't throw to her as much as i threw to my brother. Sure, i get it that it wasn't nice because she's my friend and all, but its not like i MEANT it. So i was just wondering, why do you think people fight? Is it because they like the drama? Or because their emotion are raging and take over? Especially about the stupid things! So tell me, why do you think people fight?