Background
Scrubs is a recent medical school graduate, just trying to pay off his student loans. Having been turned away from the few hospitals he has worked at for his terrible bedside manner, he is now struggling to keep up with payments. Although a talented doctor, he cannot pay with simply house calls and charity work.
As such he has turned to working as an on-call doctor for criminals, just until he can work out his personality well enough to begin working an honest life in the medical field, and pay off those ever-haunting loans.
Always studying and always short on sleep, he is irritable and abrasive at best. His lack of patience and condescending manner can make him difficult to get along with, but his work with a medkit makes him worth keeping around. That, and finding him curled up and asleep in strange locations can make for a good laugh.
Motivation
Become the greatest doctor he can be, and pay off his student loans.