Since the sides of the belt are used for transmission of power, the sides have very large surface areas. The reason that the belt does not slip is because of the wedging action of the belt as it curves into the pulley grooves. Because your belts are so essential to so many parts of your engine, it is a very good idea to periodically check their condition. Check for cracking, splitting, or fraying, especially before summer. Also, check the tightness of the belt and have it adjusted according to your owner's manual specifications.
Belts have a tendency to loosen with use.
On the other hand, you don't want the belt to be too tight, or it will put too much pressure on the accessory bearings and cause them to die an early death. If a belt is over three years old, have it replaced even if it looks good.