But perhaps an even bigger reason is simply my need to prove something, both to myself and to the world. Growing up fairly poor in a rural part of the South, and being the shy, awkward, geeky kid through most of my school years, I felt a touch of resentment for the guys with the fancy cars, the pretty girls, blah, etc. Not to mention the rich people in the fancy house who lived around where I lived, but who were in a different world in a sense. I - even after all these years - still feel a need to prove that I'm just as good as they are, and just as capable of achieving whatever I set out to do.
And I'm still pissed off about not getting voted "most likely to succeed" in my high-school senior class. :-)
Seriously, if you really like what you do motivation comes by itself. If each time you think of something related to it you go "yeah, awesome". If each time you improve it (by polishing the idea in your mind or over email/irc with your partners, or by adding a feature, or by fixing that bug) words like "awesome" come to your mind, then the motivation is auto-generated. But I truely believe this can only be acheived —at least if you're honest with yourself— if the thing we're talking about is the top idea[1] in your mind.
(A lot of "rationalization" is, I've found over time, only so much hand waving.)