When you are doing something that you really want to do, you are in a state of self-motivated self-discipline.
At that time, you are doing what you want to do and you don't feel that what you want to do is painful.
But if you are doing something that is out of your way, that is not what you really want to do, then you probably have to struggle and you have to work very hard to get your focus there.
In those latter cases, I think you will start to experience a kind of unreasonable force, or suffering, or pain, because you are doing something you don't want to do, even if it is the same training.
If you are doing what you really want to do, the process is a proper training or discipline, and it is smooth and without any suffering at all.
Training or discipline is simply a matter of focus.
Bashar.
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