One too many hangovers, and being sober gets to be a way more appealing lifestyle. It may be hard to get going, but after a while your body will adjust. I do feel that most people need something different to fill the void(for lack of better word) or some other sense of feeling good to replace the euphoria of drunkenness. Exercising works well. a new hobby, or relationship are other popular choices. A lot of it is your body's chemicals and reaction, but I'm a firm believer in mind over matter.