They say that because when someone does something wrong to someone else then eventually that will come back around to bite them back.  If you commit a negative action against someone then chances are they will not forget it and if the act is bad enough they may just want to get revenge.  Or they may tell someone else about what you did and that can cause a chain reaction of negative events against you.

The same goes for positive things.  If you help someone out.  They usually will not forget it and they may tell someone else about the good thing that you did.  This could also lead to a chain reaction of good things for you.  The more drastic the event, the more drastic the result.

Well, it is time for Karma to return all those good things I did for other people in the past.  Some people turn to God.  Not me, I am turning to Karma.  Come on Karma, I could really use your help right about now.