Happened to me last spring. An elongated vertical dent in the rear passenger door. To answer the original question about why people can't be carefull with their doors:
1. They aren't one of the few percent of people who actually car about their (and other people's) cars.
2. It was truly an accident.
3. It was a little kid that did it, doesn't know any better and if the parent saw it, they'd rather flea then take responsibility.
4. Someone is jealous. You have something they can't and rather then deal with it they will destroy it, seems to make them feel better or something.
5. You could have done it and didn't realize it... NOT VERY LIKELY!!!!!!!
6. People are just assholes!

1. They aren't one of the few percent of people who actually car about their (and other people's) cars.
2. It was truly an accident.
3. It was a little kid that did it, doesn't know any better and if the parent saw it, they'd rather flea then take responsibility.
4. Someone is jealous. You have something they can't and rather then deal with it they will destroy it, seems to make them feel better or something.
5. You could have done it and didn't realize it... NOT VERY LIKELY!!!!!!!
6. People are just assholes!