I don't understand people who believe in karma.

Why does it have to be a cause and effect situation where it keeps you from mistreating someone just to prevent bad things happening to you? Why can't you just treat someone well for the sake of being a decent person? There shouldn't be an incentive.
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"What I do know is that if Karen Carpenter and Mama Cass Elliot had shared that sandwich they'd both be alive today." -Michael K