I think that the key to this is in Bounty Hunter. There's one key line in it that can be traced to his decision.
You won't live forever...
Or something like that. I think it's when Jango's just stolen the Slave 1. Based on this, I think that his reason for getting a clone is a tribute of some kind to Roz. If he got a clone, he could technically live forever. I think that this one line sparked his will get create Boba.