Justin Taylor just posted two really great quotes from C. S. Lewis (both originally from Lewis's Letters).
"(Sensual love) ceases to be a devil when it ceases to be a god. So many things--nay every real thing--is good if only it will be humble and ordinate."
"When I have learnt to love God better than my earthly dearest, I shall love my earthly dearest better than I do now. Insofar as I learn to love my earthly dearest at the expense of God and instead of God, I shall be moving towards the state in which I shall not love my earthly dearest at all. When first things are put first, second things are not suppressed but increased."
I think these quotes contain a wealth of insight into the nature of sin. Sin is making a good thing into an ultimate thing, thus displacing God himself as first and best. Sin, at its heart, is idolatry - what Don Carson calls "the de-godding of God." When God is de-godded, everything else falls apart too. But when God is central, every other good takes its proper place in relationship to him.