Riddle: Solve For Monday

A man lives on the twelfth floor of an apartment building. Every morning he wakes up, gets dressed, eats, goes to the elevator, takes it down to the lobby, and leaves the building for work. In the evening, he goes through the lobby to the elevator, and, if there is someone else in the elevator (or if it was raining that day) he goes back to his floor directly. However, if there is nobody else in the elevator and it hasn't rained, he goes to the 10th floor and walks up two flights of stairs to his room. Why?