If you train very intensively, you need adequate recovery between training sessions.
Exercise puts positive stress on the body by forcing it to function outside its comfort zone. During rest, the body will make adjustments to better face the challenge next time you exercise. These adjustments are what will make your body stronger. In other words, fitness improves not during exercise but during rest. Giving your body ample time to recover from exercise is therefore crucial to getting fit.
Disregarding the need to rest and recover is quite common practice and may lead to injury, overtraining or fatigue. Resting properly isn't the same as skipping workouts or being lazy. Resting is about giving your body the time it needs to get stronger and fitter.
Suggestions for recovery:
Take one or two days off per week from exercising
Follow a hard workout day with an easy day
Don't be afraid to take a day off or reduce training if you're tired