Like Brock's firefighting partner Amy, he is constantly at Brock's side while he is in hospital.
Derek admits to Cameron that he's in love with Amy, but that she's engaged to his brother and he can't do anything to hurt them. Based on this information, House diagnoses him with broken heart syndrome and they treat him with electroshock therapy. After treatment, Derek doesn't remember either Brock or Amy, but they both agree to take care of him.
However, the team later talks to Amy about her engagement, and she denies being involved with Brock. The team realizes that Derek was delusional and that his problem lies elsewhere.