Lateral Thinking Puzzle

#21

WHY ???

A man resides at the 17th floor and uses lift everyday. While leaving for office he reaches ground floor directly and follows to office. In the evening when he returns, if he is alone, first he will go to 10th floor and walk by stairs for the remaining floors, else if there are other people also with him or it is a rainy day, he manages to reach directly at the 17th floor. Why does he use such a technique?

  Difficulty Level - 2/5

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