Every time we think or do something, we create a cause, which in time will bear its corresponding effects. And this cyclical cause and effect generates the concepts of samsara (or the world) and birth and reincarnation. It is the personality of a human being with its positive and negative actions – that causes karma.
Here’s an is American way of saying that Karma happens. Echos come back. It’s hard to reap what you didn’t sow. We are all cut of the same cloth. These are just the basic facts of humankind.