Why does my hair grow so fast?

It's not really my whole head, but my bangs. They grow so fast that I end up having to cut them almost every other week. I cut my hair myself so it's not that huge of a deal but it's kind of obnoxious having to cut my hair all the time. I'll cut them and then in a week or two they'll be in my eyes again. So that's about half an inch to an inch that they grow in such a short period of time. Not to mention I straighten my hair every day and use a lot of hairspray...so I don't get why they grow so fast. I'm not really looking for a way to make them stop growing so fast, I'm just curious as to why they do grow so fast.