Let me first try to explain that how and when muscles get build before I try to explain overtraining and its negative effects on bodybuilding. I would say that majority of the bodybuilders think that when they train their muscles in the gym and put themselves under lots of efforts in the gym, it is the time when they are getting bundles of muscles. Thinking this way, they try to train harder and harder and increase the frequency of muscle training as well thinking that they are going to get more and more muscles this way which is WRONG.
Let me now try to explain that how muscles exactly get build. When you train your body in the gym, training causes your muscles to get damaged and it tears them up. When this happens, your body demands certain amount of rest and gap in the exercise to make sure that it redevelops the muscles well.
Now let me explain what is overtraining and why it needs to be avoided? Overtraining happens when you either train your muscles much harder with higher frequency level or when you do not take proper rest. Why overtraining is not desirable because you must have heard that excess of everything is bad. In an effort to get muscles quickly, you can really slow down the muscle development process by overtraining. When you over train your body, muscles get damaged to an extent that their recovery and development process gets affected and they take more time to repair and growth slows down. Overtraining not only causes slow muscle development process but in certain cases it also causes various injuries to the body.
You must not train same muscles more than 3 times a week. You should take rest ranging from 7-8 hours a day to ensure recovery process gets speeded up.