Why do people drop litter?

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Our streets, highways and beaches are increasingly being fouled by the litter and rubbish that people have dropped rather than finding some other means of disposal. I have seen instances of children dropping sweet wrappers in full view of their parents without being told off for it - and the parent then dropping their empty cigarette packet on the street shortly afterwards. I once saw a young man finish a can of Coke then drop it on the street - he was actually sitting on a litter bin at the time! This is a mentality that I simply cannot get my head around, because I have never knowingly dropped a single piece of litter in a public place in my entire life - should this happen by accident I will always run after it and pick it up. It is just automatic to me to take my rubbish home or find a bin to put it in. So why am I such an apparent rarity? Do other people really want to live in a filthy world?