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.
Sin affects men in various ways, but it attacks them in every facet of life. It hinders their walk with God and hurts their ability to reach others for Christ.
People in the world are generally quite self-centered. Even saved people are often lovers of their own selves instead of preferring one another.
