Just as thinking negative thoughts will bring about negative results. A heard a story about some friends in academy who decided to play a joke on one of their buddies. And he went and looked in a mirror. Maybe I'm a little pale. He went to see the school nurse and went home and did not come back for three days because he had a genuine cold that was put onto him by his friends in his mental attitude.
Negative thoughts can bring about negative results.
This also works in the opposite manner. Scientists have been studying the effect of laughter on human beings and have found that laughter has a profound and instantaneous effect on virtually every important organ in the human body. Laughter reduces health-sapping tensions and relaxes the tissues as well as exercising the most vital organs. It is said that laughter, even when forced, results in beneficial effect on us, both mentally and physically. No wonder Paul writes about joy.