I don't think startup people are "quitting their boss" (i.e. running FROM something), but they are rather running TO something that is magical and not entirely rationale. Statistically/financially, it's a dumb decision to start a startup-- but it doesn't matter. You can sit down, run the numbers, make pro/con lists-- but it doesn't matter. You've already lost. Here's what you can't match:

1) The big win. You can heap stock options on him, but your board will block you from giving a dev more than 1% or so, I presume.

2) His baby. His own company. A sense of TRUE ownership.

3) A clean slate. New companies/new products are more fun than existing ones for a lot of people.

4) Having ANY boss. Everyone rolls their eyes at even the best boss SOMETIMES. The irony is that, if he succeeds, he'll be that boss. But it's a great feeling to NO MATTER what, be able to fix something that organizationally broken if you think it's important. To be where the buck stops.

5) Fame/notoriety. Some people are motivated by this. If your startup wins, he'll be an unnamed dev in a winning startup. If his startup wins, he'll be the guy the led it there. Either way, it feels WAY cooler to say "I own my own company" when someone says, "so what do you do?".

Doing a startup isn't a sensible CAREER decision... It's a largely emotional lifestyle decision.