There's a lot of reasons for this.

I think the biggest one comes from a fundamental law of the universe: an object in motion stays in motion, an object at rest stays at rest.

In a word: momentum. Our natural tendency is to keep doing what we are doing until a "force" acts on us in such a way that we are compelled to move or change direction. We see this a lot with procrastination. We keep putting off something until the deadline hits, and then the weight of that deadline forces us into action. Ever notice that after that happens, once you break through those first couple of minutes of not wanting to do a think, that it suddenly isn't nearly as bad as we thought it would be?

I get this all the time when it comes to doing dishes. My dishes pile up in the sink and the thought of doing them is almost unbearable (I hate doing them). Then, I force myself to go do them and by the time I have the water running, I'm actually enjoying doing it (because I listen to a book on tape or music while doing them).

So, to get past this we have to find ways to overcome the momentum of inaction. One of the most effective ways, I think, is to set goals so tiny that you can't find any reason NOT to do them. For example, if you're a writer, set the goal of writing 100 words. Anybody can write 100 words (it'd take even the worst writers ten minutes to do that).

You'll find that once you start typing those words, and once you reach that goal, that you won't want to stop. Often a 100 words will turn into 2000 words simply because of the momentum. Once you've started, you won't want to stop in the middle of it.