Okay so this guy lives on the 17th floor, he goes all the way up when it's raining or when he's with a friend. But if he's alone it the sun is shining he rides up to only the 9th floor and walks the rest if the way up. Why is that?
For the exercise? Maybe that's all he can do right now. Maybe he is working up to walking the whole way. As for the sun shining. I don't know. Does the stairwell have windows?