It is amazing the satisfaction that achieving something can bring. I know every time I step on the scales and see that I have lost weight it gives me a great of achievement and spurs me on to lose more (and I really need to). I suppose it is the fact that you have done something productive that gives you such a high. It is the same when I land an acting job. It is such a boost to know that I have managed to do something, that someone else has seen my work and decided to pick me out of all the available people, and I am sure the same goes for any walk in life. I have heard that selling an article for publication gives a similar high and encourages you to write more and seek that achievement fix once more. However, it doesn't matter what the achievement is, big or small, you stick get a kick out of hitting that goal. Maybe it is taking your first steps, or learning how to make an omelet, or going on holiday on your own. They may be small things to other people but they are big things to you and doing them gives you a craving for more.