Many people fall victim to the 'no days off' approach or talk themselves into training everyday despite what they might hear about the benefits of rest days because they claim they don't feel good when they take a rest day.
So the question is; as long as you're up for it, is it really so bad to train everyday? Regular exercise has so many benefits it's not even worth trying to list them all. But it's less widespread knowledge that exercising TOO much can actually reverse the benefits of regular training and even cause the opposite effect. Even if you manage to make training such a strong habit that you don't feel the need to take days off, there may be some negative impacts to disregarding rest days. First off, rest days provide a psychological break from the grind of hard gym sessions. Without them, many may find it hard to turn up and feel motivated enough to put in a good effort during their training sessions throughout the week because they are constantly burnt out or bored from going to the gym everyday.
'But I love to train!' If you don't feel the psychological burden of training everyday (yet), if you are putting an adequate effort into training, the longer that you go without resting the higher your risk of overtraining. In short, overtraining is when one places enough stress on the body that the demand is exceeding what they are able to recover from. You only benefit from the training you effectively recover from.
Overtraining can be a temporary condition, but if you continue pushing through it long enough, it can place enough stress on the body (both physically and internally) to cause more serious, chronic damage. Overtraining can have negative impacts such as (but not limited to) overuse injuries, loss in strength, disruption to sleep cycles, change in appetite and mood, irregularities in menstrual cycle, irregular heart beat, chronic fatigue, etc. How do you know if you're over trained? If you train hard more than 4 times per week and experience more than one of the above symptoms, it may be worth looking into further. If you train everyday but DON'T have any of the above symptoms or feel over trained, you may want to take a step back while you are ahead and learn to respect your body's need for rest and recovery before you run yourself into a more complex problem than the psychological (and likely irrational) fear of taking a day off.
How Many Rest Days Do You Need? The exercise threshold varies widely from person to person - so don't feel that just because someone else feels fine training hard 5-6 days a week, you are a failure for feeling tired from training more often than 3-4x/wk. Ability to recover from training can be affected by many factors, from stress levels to workout intensity to diet to sleep to (training) age and even genetics. It's recommended that you take at least one day off each week no matter who you are - if not 2-3 days for optimal health, recovery, performance and results and to maintain a high quality of life and enjoyment of your training.