Jason began showing anxiety and stress in the earlier episodes of the second season as a medical student. He was caught drinking on the job and often got too emotional and stressed over the tougher cases.
He constantly asked Sarah to give him some prescription medicine for his stress, but she always told him she couldn't without a formal discussion with him and wasn't allowed to just prescribe, which is why his death hit her particularly hard, as she knew he was struggling.
In Monday Mourning, he came into work happy, saying 'good morning' to everyone. He then went to the roof of the hospital and jumped off.