A lot of gym-goers worry themselves about 'overtraining.' This term refers to the idea that, by working your muscles too hard too frequently, you can actually hinder your strength gains as your muscles never have time to recover, repair and grow.
It can almost become a negative, demotivating force in itself. Have you ever been on a roll with your training, perhaps working out 5-6 times a week, and a well-meaning friend has commented "oh, you're doing too much, be careful of overtraining." Suddenly, the seed of doubt is planted inside your mind, and you can no longer put your heart and soul into your training and go home feeling so good about yourself, because at the back of your mind is the question... am I doing more harm than good? Am I overtraining?
Am I overtraining?
It's important to realise that for most people, psychological overtraining will kick in way before muscular overtraining ever does. And that's ok. When you get that feeling of demotivatation where your enthusiasm for the gym begins to wane, don't let it permeate your entire being. It really is important to listen to your body (and not your mate who keeps warning you about overtraining), so if your body and mind have had enough for a day, two, three, or even a week - that's all right.
The aim with any training programme is surely for lifelong strength and fitness. Unless you are training for a specific event (in which case, presumably, you're motivated enough by this goal that this phenomenon does not apply to you), the most important thing is maintaining some element of resistance and cardiovascular training in your life for the long term.
Many hardcore fitness and bodybuilding sites will tell you to push through the mental wall, and force yourself into that gym. If your alternative is moaping on the sofa feeling guilty, then yes, do that. But if you have a project you need to work on - some other productive use of your time - don't always sacrifice this for gym time. Mental health is important, too. Your brain also needs some exercise and recreation, or you risk suffering from psychological overtraining far more than you do muscular overtraining. And when you do return to the gym, you'll be raring to go. Intensity > volume, always.
Why physiological overtraining is unlikely for most people
Unless you are disciplined enough that you really could get yourself to the gym twice a day, every day, for a month or more, muscular overtraining isn't likely to be a problem for you. That's right. The body can even cope with two-a-day gym sessions, even on the same body parts, provided you give it sufficient rest time in between - but crucially, it's a short-term strategy "for those times when you are extremely motivated to train and the idea of going to the gym twice a day sounds appealing" (International Sports Science Associaton, quoted on bodybuilding.com)
For 99% of us, this won't be every day for an extended period of time, so don't live in fear of overtraining.
Listen to your mind
So whatever phase you are currently in - can't get enough, or can't face another workout - listen to your body and your mind. Take advantage of your fitness motivation while it's there, and don't suppress yourself due to fears of overtraining. Similarly, if you need a little downtime, take it. Your mind and body need it, and it's far better to sacrifice a couple of days, than lose motivation altogether and sacrifice months or years.