Why do people still do something that they know it's wrong?

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In china people will take some plastic bags from salespersons to pack things after shopping .Most of people know that the plastic bags aren't good for environment,but they still to use them and sometimes throw them here and there. Some people know that forest is important for environment,but they still cut down the trees. Why do people still do these things? When disaster is coming ,they will truly know what they have done!!