Why does David Lurie lose his job as a professor?

David Lurie loses his job after seducing one of his students, a vulnerable young woman by the name of Melanie Isaacs. Although Melanie tries to break things off, Lurie persists in pressurizing her to continue with the relationship. Traumatized by the experience, Melanie stops attending Lurie's classes, to which he responds by deliberately awarding her bad grades. A formal complaint is made and Lurie is hauled before a disciplinary hearing. At that hearing, he refuses to apologize for his actions or even defend himself in any way. Inevitably, he is forced to resign in disgrace.
Lurie's predatory behavior is entirely in keeping with a life fast spiraling out of control and sinking into a mire of hedonistic excess. This is a man in desperate need of redemption, which he will eventually find—albeit in a limited sense—after a lengthy period spent on his daughter's farm.