Explain: Elevator Puzzle

The Puzzle:

Explain: Elevator

A man works on the 10th floor and always takes the elevator down to ground level at the end of the day.

Yet every morning, he only takes the elevator to the 7th floor and then walks up the stairs to the 10th floor, even when in a hurry.

But he will go all the way to the 10th floor when others are in the elevator with him or on a rainy day.

Why?

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