Shayari Bazar

A man lives on the twelfth floor of an apartment building. Every morning he takes the elevator down to the lobby and leaves the building. In the evening, he gets into 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. Otherwise, he goes to the tenth floor and walks up two flights of stairs to his apartment that is on the twelfth floor. Why?