On the same lines as muscle soreness, I believe that you need to "push the boundaries" every now and then. Bring yourself out of that "comfort zone".
Just like when you first started lifting, and every muscle killed for week, but you still trained through it. You trained through it because your body was simply adapting and "learning" quicker recovery.
It's easy to get stuck in a rut and convince yourself that you always need to rest a muscle 'X' amount of time.
Some can't see the benefit to training some muscle groups more than once a week because when they try it more more frequently, their muscles hurt (as if not recovered) but this goes away within a few weeks of training in this fashion.
If you push the boundaries slightly, you challenge your body to "over-come it".