It's a story I really, really want to tell. I've been carrying it around in my head for years, and committing to finally DOING something with it actually felt like a huge weight had been lifted from my shoulders (though that might have just been the pre-decision anxiety evaporating).
Of course, telling this story involves telling it to someone, so in those moments when I don't feel super-enthusiastic about it - and there are a bunch of those! - the knowledge that there are people reading it and looking forward to the next update really helps keep my spirits up.
But the number of readers doesn't really matter, I guess? I mean, I'd still keep my comic going, whether I had 10 readers or 10 000. Someone's reading, is the point. Though having a big bunch of readers is obviously a really cool thing. I don't know why, but I've never gotten over the feeling of amazement that there are people other than my mom who thinks what I draw is cool. XD
Also, in purely selfish terms, there are bits of this comic that I really want to get to draw, coming up much later in the story, and if I quit now I'll never get to do that.