Man is not promised an endless supply of strength for him to store up. Rather, God promises to provide the necessary grace and strength for the present trials.
Change often results in fear. As such, it must be met with strength. Failure to do so can breed panic that only God's strength can overcome.
There are many things in this life that are not sinful in and of themselves, but when they are misused they can be deemed as sinful or harmful.
Many people use time set aside for rest to do things that deprive of rest. This compounds the need for rest and the problems associated with not getting any.
