Granted he probably doesn't like Obama much, but if he's at all serious about his agenda he's going to have a much better chance of pushing it from inside than outside.
I can imagine press and pundits and consultants telling Obama to avoid Edwards because he's confrontational populist poison.
But outside the Beltway madness machine, it's the perfect pairing. And Edwards will know this.
Why would Edwards not accept VP?
I can think of two reasons: he's been there, done that and ego.
Offsetting these is the fact if he doesn't accept VP and get elected he's done as a major national politician. You only get so many chances at the big prize before you get elbowed aside.