Cotton is great...if you are in a warm and dry area. In humid areas, it never dries out. Synthetics generally dry much faster and chafe less when they do get wet. Also, if it is cold, wet cotton will not keep you warm, but wool can keep you warm even when wet, an important consideration if you are exercising (and sweating) in cold weather. I've also noticed that sweaty cotton tends to start smelling really bad sooner than sweaty wool/synthetics. And wearing the same sweaty chamois two days in a row does suck, but its a hell of a lot better than wearing the same sweaty boxers two days in a row.