I agree with it 100%. It's hard to tell when to quit and when to stick with your startup/product, but not having fun for too long is a sure sign you should be doing something else. I currently have the same problem, not having fun anymore doing it. Part of it not being fun is the market I chose, selling software to people I have a hard time relating to (small business owners in France, not familiar with the Internet and web apps).

For my next product, if it's B2B, I'll make sure to first find an audience, people I'd like to do business with, and then find a product to make (incidentally, that's also the approach advised by Rob Walling and Amy Hoy among others, and it makes lots of sense to me now).