A Man In The Elevator

I found this little puzzle and it really puzzled me, perhaps you would like to try solving it…

A man lives on the tenth floor of a 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 seventh floor and walks up three flights of stairs to his apartment (even though he hates walking). Can you explain why?