There's another type of depression that comes from always grasping for (money|acceptance|fame|success|whatever) and never fully attaining it. For people stuck on this treadmill, the long perspective can be a good reminder that none of it is really worth getting too bent out of shape about. On a cosmic timescale, all these things we worry about will be gone and forgotten in the blink of an eye.

The best antidote to avoid swinging the other way, into nihilism, is to simply live in the present as much as possible and enjoy what's there. Things don't need to be permanent and important to be enjoyable. Think about games. We know the outcome doesn't matter in any larger frame of reference, but we still like playing and get wrapped up in them for their own sake. Life is just another game--not ultimately significant, but fun to get wrapped up in if you don't take the outcome overly seriously.