Why does the Christian life involve discipline but not just giving things up?
The Christian life involves discipline because through the Spirit one mortifies the deeds of the body to live. Yet it is not just giving things up, since the change is a matter of changed appetites: what was once loved becomes hated, and vice versa, so obedience flows from a new desire, not mere external surrender.
Les teaches that Romans 8:12-13 says believers are debtors not to the flesh, to live after the flesh, for living after the flesh brings death—the same admonition given to Adam and Eve. But if through the Spirit you put to death, or mortify, the deeds of the body, you shall live. The Christian life does involve discipline, but as Les stresses, you aren't going to give up things you just love to do, because the things you once loved you begin to hate and vice versa. So it's not a matter of giving it up; it's just a matter of having a change of appetites.
For if ye live after the flesh, ye shall die: but if ye through the Spirit do mortify the deeds of the body, ye shall live.