That was the big reason for me. There were a bunch of contributing factors - rejected by YC 3 times, no luck with other outside investors, better opportunity came up, all other founders quit, scope of the project was much bigger than we envisioned, continuous problems simplifying the UI enough that I'd want to use it myself, and some doubt about whether current browser performance is even good enough to run the product - but I kept going through all of them until I really had no idea what to do next.
Joel's comment about it taking a long time before you can even know what knobs to tweak is right on. Early-stage products are almost completely wrong, and oftentimes they're so fractally wrong that nobody will bother telling you how to make them right. Launching helps, if you can get users. But many startups fail to get even one loyal user.