People believe that because you do a good deed, something good will happen to you and vice versa. This is somewhat true, however, it isn't because of some powerful energy that believes the world shall be balanced, but rather the ripple effect...let me briefly explain:
Let's say you do some good deed to a random person. they will take notice and more than likely, they will do a good deed for a different person. this is a chain effect and the chances of it that one good deed coming back to you are slim.
Now lets say you did multiple good deeds to random people. the chances are higher that one of those chains of good deeds will come back your way, thus the people who believe in karma live life in a more positive way, do more good deeds, and have a better chance of receiving a nice gesture in return.
People will also take not on the actions of said person and realize that if anybody deserved a good deed, it would be him/her. this will take place of the chain of good deeds that never reached to you.
This has the same effect for debby downers they will create more negative chains rather than positive chains in the world, therefore receiving more negative gestures and nobody will feel they deserve a good deed. instead, people would think that they deserve only the worst.
This is what we call karma.