I agree with what's been said. You need a "..."

Can you truly get behind an idea that's not your own... if you have another reason for being there?

Yes.

I want to start my own company (based on my own idea) some day, but I'm learning hell of a lot (and saving up cash) working at another company, on someone else's idea. Getting behind that idea, even if I'm not necessarily passionate about it, is important, because that's the only way I'll do a good job and actually learn what it's like.

So yeah. I think it's important to gain some experience before setting out on your own. It's a safer route if you don't have people to fall back on if your first (or second or third) attempt fails