Having fun has some great side effects:
Being yourself. I dare you to have fun while being dissatisfied with yourself. I'm going to say that it absolutely cannot be done.
Better health. There is a whole list of health benefits, but just to get you started, lowered blood pressure, stronger abs, and boosted immune systems are among some of the benefits.
Better sense of community. It's pretty hard to have fun with someone who is not a friend. It can also work backwards--if you intentionally have a great time with someone you feel neutral about, you are well on your way to becoming great friends.
A more resilient temperament. If you can laugh off your mistakes and the blunders of others, you're in a much better place than if you simply hold on to grudges.
These four side effects hardly even begin the list. What are some other benefits?