Why do people drop litter?

I was taught as a child not to drop litter, either take it home or put it in a litter bin. In my local park - it's a very big park, some kids must have had a party in the evening and left bottles, cans, fast food wrappers etc. scattered around where they partied, why, what is the mindset, some of this stuff was about 20 paces away from a litter bin.
Another is people whom throw stuff out of their car window. I'd love to know what their houses are like. They must believe in the tidy up fairy.