I love Timothy Keller's sermons because he takes the central pillars of the Gospel--the basic truths we take for granted--and finds treasures in them that we've never noticed before.
Here's the written intro his site provides for this sermon:
"Tim Keller sermons via Gospel in Life: A lot of us are cast down because we don't expect the attacks on our peace and joy that are inevitable. Much of the depression we experience is depression over our depression. We're sad that we're sad. We're surprised that we're surprised."