The Man in the Elevator

The puzzle

A man lives in a high-rise apartment building. He has an unusual custom regarding the elevator upon his return home from work each day. If it has been raining, he always takes the elevator to his floor, the 17th. If it has not been raining, but someone else is in the elevator with him, then he also takes the elevator to the 17th floor. But if it has not been raining, and he is alone in the elevator, then he takes the elevator to the 11th floor, and walks up the stairs the rest of the way. Why?