Why do fan belts break?

The fan belt on my car which I share with my sister recently broke, and now I'm having to fork out �40 towards the cost of it getting fixed. I'm at uni at the moment, so she's been driving the car solely for 3-4 months. What I want to know is if it's her fault? I'm guessing that it's not from my little knowledge of cars, but she's also managed to break the lock on the main door and I know that she's skidded in the car before from when she wasn't driving properly, so I'm just curious to know if she's also to blame for fan belt breaking... :P