solve this riddle

[toef]

Here's another classic riddle....


A man lives on the 10th floor of his apartment building. Every morning when he leaves for work, he gets in the elevator, hits Floor 1 (the ground floor) and when the elevator takes him down, he walks out and is on his way.

After hours at his boring job, he always goes straight home. If it's a rainy day, or if other people are in the elevator, he always takes the elevator up to the 10th floor. Otherwise, he always takes the elevator to the 7th floor, and takes the stairs for the final three flights.

Why?