I'm mad at people for using the elevator to go up one floor in a 20 floor building.
Does it happen often at your place of work?
More than I can count.  I am tired of having to walk all the way up when other people could just walk one floor.
I understand why you're annoyed, but you shouldn't judge others for using the elevator, even if it is only for one floor. You don't know if they're injured or have some condition that makes it difficult for them to walk up stairs. I think you should try to be more understanding about what other people might be going through.
But it's still annoying and I don't like it.
It might be good if you think of walking up the stairs as exercise. That sort of activity is really good for you, and can help keep you healthy.