At the risk of sounding BarneyStinsonian my answer would simply be "'cause it's awesome!".
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.
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At the risk of sounding BarneyStinsonian my answer would simply be "'cause it's awesome!".
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.
[1] http://paulgraham.com/top.html
Awesome! :)