Hmmm, maybe there is this magic that actually does make a bond between wizards that owe the other wizard. Or maybe Dumbledore was being himself again (God, he does that alot) and it's not really a magical bond but more of "the goodness that's in everyone" that Dumbledore's talking about.

So maybe, here's my point finally, Peter doesn't really HAVE to help Harry at all, but it's just that Dumbledore believes he will.

Because, we all know, Peter would never defy Voldemort because we all also know that Voldemort would just kill him there on the spot, and Peter is and never was that brave.