Personally, I think it has a lot to do with our perception as children and then as adults. If you think about it, a child sees the world as a place of discovery, most of our early lives are spent learning by finding new things, places and there is a sort of "newness" in each day. When we get older, that approach gets lost in routine and mundane tasks. Somewhere, we lose the ability to really see each day as a potentially interesting experience and before we know it, weeks become months then years and then decades until we snuff it.
Paradoxically, how many times have you heard somebody say something like "that last 10 minutes seemed like an hour"?