Half the Jury Is In

This question really gets to the heart of Buddhism very quickly.

Perhaps I am making things too simplified here, but here is the skinny on this one.

Karma is tied up with dependent origination (pun intended).

When you focus on a set of circumstances, conditions, et al, then you propagate those conditions. You strengthen and reinforce them, connecting and adding to them with similar conditions and circumstances.

As all is connected (dependent origination), you are shifting your attention selecting more and more of the like circumstances into your experience.