I can describe my personal experience and comment on the experiences of those I've read about.
After Victus Media failed to lock onto a product with traction both my cofounder and I let it go. We iterated on several concepts between October 2009 (I started with an idea late summer 2009) and mid 2010 (for more detail visit http://victusmedia.com).
We were unfunded and did all design and development on our own dollar so there were no financial partners that we had to work with on shutting down the company, or employees that we had to let go. It was an incredible learning experience, and I've never felt that surge of motivation before while working for other companies.
I'm currently working part time at my first career (simulations/algorithm engineer) to pay the bills while I continue to learn and practice developing side projects at nights and days off. I tried applying to a couple dozen startups in NYC, but I don't have the specialized experience they were looking for. My plan is to experiment with different ideas, learn the required technology, and explore new market opportunities. The criteria for me to invest heavily in new projects is having adequate resources^ to build an alpha product which resonates strongly with folks that try it out.
In regards to other entrepreneurs, nearly every successful founder has a trail of failed ventures behind them. With each effort they learn or continue to fail. I have great respect for the generous folks that document why they believed their startup failed. The cause of failure can almost always be traced back to failing to listen to your target audience, failing to lead the market through tough design decisions, or failing to execute fast enough.
The founders I've met and interact with regularly are hard wired to excel in the ill defined and open space of startups. Sure they could take a specialized job working for another company, but their skill sets and more importantly their passion will always be better fueled by working on a business they have a vested interest in and influence over.
*= My favorite quote from Roger's answer
Don't let failure color your perceptions of yourself. Take failure as a valuable learning experience that is merely a step on the road to future success. Whether you do another start-up or not isn't the point; it's to use the invaluable data you've collected about how businesses work - or don't work - that you can take with you for the rest of your working life
^= available resources consist my own coding time, operating partners and team