I find I suffer from a similar problem. For me I figure the reasoning is based in learning potential; at the beginning of a project or piece of software you have the most to learn. As the project continues you begin to do more and learn less, and since I enjoy learning new things a lot I find myself less motivated to continue and more enthused to start something else (which also will not get finished).

It's a vicious cycle of incomplete work and hyper-learning, but I'm mostly okay with that.