Daniel Kahneman in Thinking, Fast and Slow:

I had stumbled onto a significant fact of the human condition: the feedback to which life exposes us is perverse. Because we tend to be nice to other people when they please us and nasty when they do not, we are statistically punished for being nice and rewarded for being nasty.

There reason for that lies in regression to the mean when training (example of flight instructors in the israel airforce):

I pointed out to the instructors that what they saw on the board coincided with what we had heard about the performance of aerobatic maneuvers on successive attempts: poor performance was typically followed by improvement and good performance by deterioration, without any help from either praise or punishment.

Since positive reinforcement is so counterintuitive: don't forget to reward yourself for rewarding somebody for good behaviour! :)