I highly recommend reading "Practical Programming for Strength Training" by Dr. Kilgore and Mark Rippetoe. The first few chapters will help you better understand what happens in your body as you exercise. The part that applies to this answer is that the 48-72 hours of rest applies to beginners and novices, not to intermediate or higher athletes.
A program designed for each class of athlete has a certain amount of stress applied to cause you to adapt and be better at your exercise. Between each period of adaptation inducing stress is a period of recovery. For beginners and novices, recovery happens in the form of rest. For intermediates and higher, recovery is composed both of rest and "active recovery"--a lighter stress workout to keep the volume up, but not overtax the recovery process.
- Beginners need 24-48 hours to rest
- Novices need 48-72 hours to rest
- Intermediates need up to a week
- Advanced athletes need up to a month
- Elites need several months or years
If you are no longer a novice at your sport (or with your training), you may be on your way to becoming an intermediate. If so, you'll have to make some modifications to your training program to help you keep improving.