Background
Nej Haren was a simple transport pilot for the Empire, a bus driver. Until he was caught in a battle between Imperial forces and the Rebellion. Badly injured, Haren barely survived and relegated to only basic medical care because of his rank, he decided to leave and start again far from both the Empire and the Rebel Alliance. Technically a deserter from Imperial service, the Empire won't necessarily send someone after him, but if he's recognized or triggers any alerts, they'll do what's necessary to bring him to justice - Imperial justice that is.
Motivation
Ambition/Freedom. Haren just wants to get away from the whole ridiculous & dangerous Rebels vs Empre thing and be able to do what he wants.