[euccastro]

For a more broadly targeted, more nuanced version[1] of this I found The Dip, by Seth Godin, quite helpful.

Like this essay, the book gives much advice of the "don't give up" type. But the book also mentions some cases when you should quit: you should quit activities that are a cul-de-sac, those in which you are not committed to become the best, or that don't help you become the best at what you most want to do. The rationale is that current society rewards the very best, ignoring the rest, out of proportion with actual quality/merit. I'm oversimplifying of course.

Obviously, PG doesn't think any of the YC startups is in a cul-de-sac, so he can dispense with that exception and tell all of them to just push forward. If anything, he warns the folks not to 'unquit' school and other attention/commitment escape valves.

[1] Edit: I was focusing on the "don't give up, don't lose focus" part. This essay has other big theme, shame as a motivator, which is not touched in The Dip.