This comes directly from Paul’s http://www.paulgraham.com/startupmistakes.html
18. A Half-Hearted Effort
The failed startups you hear most about are the spectactular flameouts. Those are actually the elite of failures. The most common type is not the one that makes spectacular mistakes, but the one that doesn’t do much of anything—the one we never even hear about, because it was some project a couple guys started on the side while working on their day jobs, but which never got anywhere and was gradually abandoned.
Statistically, if you want to avoid failure, it would seem like the most important thing is to quit your day job. Most founders of failed startups don’t quit their day jobs, and most founders of successful ones do. If startup failure were a disease, the CDC would be issuing bulletins warning people to avoid day jobs.
Does that mean you should quit your day job? Not necessarily. I’m guessing here, but I’d guess that many of these would-be founders may not have the kind of determination it takes to start a company, and that in the back of their minds, they know it. The reason they don’t invest more time in their startup is that they know it’s a bad investment. 
I’d also guess there’s some band of people who could have succeeded if they’d taken the leap and done it full-time, but didn’t. I have no idea how wide this band is, but if the winner/borderline/hopeless progression has the sort of distribution you’d expect, the number of people who could have made it, if they’d quit their day job, is probably an order of magnitude larger than the number who do make it. 
If that’s true, most startups that could succeed fail because the founders don’t devote their whole efforts to them. That certainly accords with what I see out in the world. Most startups fail because they don’t make something people want, and the reason most don’t is that they don’t try hard enough.
In other words, starting startups is just like everything else. The biggest mistake you can make is not to try hard enough. To the extent there’s a secret to success, it’s not to be in denial about that.