People generally are completely oblivious, and don't respect others' property. In some cases I think it really is a matter of envy. They can't stand that someone else has a nice shiny new anything while they have a second hand beater. I don't commute or runs errands with my bike, so as of yet I have not had to leave it locked up unattended, but like others here become quite angry with people that can't park their cars and have been known to seek retribution against said offenders' vehicles.