Why can't the old nature and new nature agree on anything?
They cannot agree because the new nature has a completely new set of values aligned with God's righteousness, holiness, and laws, while the old nature is bent toward rebellion and unbelief, with Satan as its god. The two are total opposites and remain contrary, at war constantly.
Les explains that the old nature declares war on the new nature, and consequently the new nature must declare war on the old nature, so it is a constant battle. The battle lines are defined from the moment of salvation until death or the Rapture. He identifies the conflict as the old Adam, or the flesh, warring against the Spirit, and the Spirit warring against the flesh. These two forces within each believer are contrary and at total odds. The new nature has a whole new set of values in compliance with the righteousness, holiness, and laws of God. The old nature, whose God is Satan and the god of this world, is bent in rebellion and unbelief. Because they are two total opposites, they war constantly, so you cannot do the things you would.