Early Trials

I've been trying to sit down and figure out how to tweak the character bios for my script, The Last Archmage, and to get a solid treatment written. The hardest part is wrestling with a point my instructors pointed out to me when I pitched the idea. I need to find the character's motivations in the beginning of the script. A lot of events are set off by the apparent death of one of my characters, which leads the main character on his journey, but before that I need to figure out what the main character's motivation for learning magic is and what drives the story forwards. I'm sure everyone has experienced this but I am just getting a little frustrated because I am doing a lot of thinking and jotting down and crossing out, but adding very little to the bios so far.