I jokingly called it 'tass' (toddlers attention span syndrome), and I've had it since I was three.
Buying toys for me was according to my family the stuff of nightmares. I'd play with something for 3 minutes really excited, then see if it could be taken apart and if not toss it aside and never look at it again. And the ones that I could take apart suffered the same lot, only in bits and pieces.
It took me a long long time to outgrow that, I still have to be very careful when I am exposed to something new and shiny to stay away from screwdrivers.
In software projects I have much the same tendency, as long as it is challenging, new and I can learn it's ok. But woe the day the last bolt is screwed on to the carriage, that's when I'm in real danger to lose interest. Building is great, maintaining is not, so I try to build things in a way that they are as maintenance free as possible (which is good anyway).
If the money would be the motivating factor (or the 'desire to change the world' as some other commenter put it here: http://news.ycombinator.com/item?id=1369566), then it would be easy to keep going.
But for me the driving force is to learn, and it's hard to keep 'learning' the same stuff over and over again. So I try to frame my days and the things I have to do that are not exactly 'new' in terms of what I will learn from them, and if I can then I can usually do the job in a reasonable time.
If not, I have a tendency to make mountains out of molehills.