Usually it's one of two things that happen..either the person is jealous of your car and they damage it intentionally to make themselves feel better in some perveted way, or it's a person who just does not care a lick about cars. This is the type of person who would open their door and slam it into your car, and if you saw it happen and confronted them, they'd respond "What, it's just a car???"
Good luck fixing it....sucks to have to deal with this kind of thing.