Apart from the fact that it feels good!
When you are joyful the vibration of the Energy within your cells attracts everything of a similar vibration to you. You give out Joy – you get back Joy. And all manner of other vibrationally high (positive) things.
When you are sad (negative) the vibration of the Energy within your cells attracts everything of a similarly negative vibration. You give out negativity – you get back negativity. And all manner of other not so pleasant things. As you sow, so shall you reap.
This is the Universal Law of Attraction.
So if you want to attract good things coming your way…