I can't speak to the mental aspects of it, but wet dreams are the body's way of getting rid of excess sperm. Once you are past puberty, they generally decline as you get older. When you are 20, they may happen every few days if you haven't ejaculated. When you are 30, maybe every week. When you are 50, maybe not at all - by that age your body may be able to absorb unused sperm as fast as it can make it.

The best way to prevent wet dreams is to ejaculate more frequently than your body demands.