I hate people littering, it's not hard to put it in your pocket or hold onto it until you reach a bin. I make a point of picking up my friends litter, but not others (don't really want to touch most of the stuff people drop in my area!)
Glasgow must be commended for their supply of bins I must say, much better than any other city.
In my experience it is because the average distance before people are willing to drop the litter is so low.