There is no such thing as karma. Things happen to us not because we did so in the past, but because they just happen to everyone all the times. What people usually refer to as "karma", is the realization of what we did in the past. Life is a tremendously rich environment: full of information, full of details, full of interactions. Lots of things happen to us on a daily basis. Some good, some not very good. This is for everybody. If an individual receives an unfair treatment, something that he has done to another in the past, his intellect recognizes that. He suddenly "realizes" how the other person felt at the time he did so to him. Hence the subject suffers twice, one for being treated unfairly now, and one for treating another unfairly in the past. And the latter part has the bigger and deeper share of the suffering, as it disturbs the image of self, that we need for the well-being of our minds. This extra suffering resulted from such realization I call karma.