Overtraining is a horrible thing to see, all your hard work will be in vain when this happens. Most of the time people eat too little of what their body needs and this cause overtraining because the body lacks essential vitamin or macro molecules for recovery. Or the most common is training too frequently, when your workout is as intense as weightlifting or more you need to rest more it is that simplke, but many people don't understand this concept, andread too many article where guys workout evceryday and never take a break, sorry to say you won't get any real result that way.