From my experience, there are a few things that can make your start-up take over your life: customers, investors, competition and personal drive.
Depending on your market segment, customers can be very demanding. If your web site goes down at midnight on Friday, you can't very well let it wait until Monday at 9:00 to put right, can you?
Investors have fronted you money on the expectation of explosive growth. If you don't deliver that growth, they might not be around the next time that you need cash.
In web-based businesses, speed is a major asset to keep you ahead of the competition. Hopefully, you've got a head start on the copy-cats, but there are more of them, and some of them might even be as smart as you.
Finally, there's personal drive -- as I think you've identified, start-ups do attract a certain personality type who aren't the kind to leave a job half-done and call it a day.
In closing, I would ask you a question -- you run a small business, but is it a start-up? PG has been pretty clear in many of his other essays that these two are not interchangeable. After all, the start-ups that are being discussed here are hoping not to be small businesses for long!