Why does my hair tangle easily?

I have thin, fine, straight hair (with a little curl at the ends) a few inches past my shoulders and at school it seems I need to brush it again after every class! I don't brush it THAT much but I do brush maybe five times a day and I know that can be bad for hair. So how does it do that? I'm basically just sitting at a desk for 50 minutes and then all of a sudden it's a little tangly and it's just not neat and smooth. So do you know why? And how can I get neat and smooth hair like most other people? I don't use a lot of products because I want to keep my hair healthy.