The biggest issue is that at a startup your whole brain is focused on what you're building. If your personal life gets involved, you'll want to make sure to make time to do other things. You don't actually want to talk about your startup all the time.

If you work 12 hour days at a startup, you probably don't have time for much else at all.

If you're a normal person who thinks that is unsustainable, you can find time for a normal life along with working on your company. That's the case whether your obsession is your kid or RockBand.

Maybe I'm the exception the proves the rule.