Why are my fish misbehaving?

I just set up my first aquarium. It is a regular size 10 gallon tank. I'm not an expert on fish, so I just picked out some fish that I though looked nice. I saw that they had a bunch of male bettas sitting alone in little jars and it made me feel sad so I bought all 35 of them and put them in my new aquarium. Now the little guys won't stop fighting! Why do you think they are doing this? They can't be fighting over a female, because there aren't any in there. What's going on?