The concept of Karma or the saying "what comes around goes around" are both statements that experience can show to be true.
I have always been a "nice" person. I have never conciously set out to harm anyone (although I have been subject to human error like everyone else). I believe that what you give to the world, you will eventually recieve in kind. The nature of open, free, kindness is that eventually you will recieve the kindness back again. Plus I have noticed that the world that you inhabit is also a product of your attitude. If you are ungrateful, there will be little to be grateful for. If you are greatful, you will notice how much you have to be grateful for plus, you will recieve more to be grateful for.
Just a little note on this, hopefully I will find the time to expound my thoughts further. As a side note, I am reading a very interesting novel called "The Magic Mountain" by Thomas Mann. It is a really good read.