Laziness is my guess. Who wants to clean or do filing or things like that when there's fun to be had doing something else? So we put it off or do it half-heartedly until it can't wait anymore. I think the trick is to do a little at a time whenever possible, and combine it with something else. When I go out, for example, I always try to grab an empty box or some other bit of garbage. I'm passing by the dumpster when I head out; why not take advantage of it? It may not seem like much, but little things like that can add up over time, and eventually the big job isn't so big anymore, and might not come back. 
