* One of the two cofounders was on his honeymoon for the month of May, right after launch.
* The other cofounder had difficulty managing everything on his own.
* Cofounder comes back from the honeymoon and decides to leave for good.
* Remaining cofounder doesn't want to run it on his own.
In my past life, I've been both of these guys and frankly, it is hard to blame either. If the passion somehow died for honeymooning-cofounder, you can't expect him to give his very best on this startup. On the other hand, if your cofounder leaves, you have very little incentive to do everything on your own. Part of working with a cofounder is each person doing what the other most probably dislikes (usually biz vs. tech). What's the fun if you have to get better pricing AND write efficient code? Most people like one or the other.
I think the take-away from this and my own life examples is to start small, not try to get too much publicity early on, work on finishing the product before making promises, and most importantly, be honest to yourself and your cofounder.