The subtitle to this work, "defeating sin with superior pleasure," says it all.  There is godly pleasure in applying the faith that first saved us in everyday circumstances, defeating the unbelief and lies that Satan throws at us.  Fighting these lies, and the resultant anxiety, pride, impatience, and other sinful attitudes that trip us up, with what Piper calls "the purifying power of God's truth" blesses us with peace as we see God enabling us to live by faith in his promises and his grace.