Because you have limitations. lack of energy, physical pain, mental or emotional pain, physical exhaustion, loss of passion and the list goes on. The trick is to be consistently flexible not consistently rigid. To not be hard on yourself when you take breaks. You are doing the best you can and you are not in a race. To listen to your body and feelings. This or you have to force yourself to do it, even if you’re tired even if you’re sick even if you lost your passion. The choice is yours always.