A few years ago, the manager of a local convenience store was about the most humorless person I ever met. She was known throughout the area for her sour disposition.
So I set out to make her laugh. It was a great self-challenge. Every time I went into the store, I tried to say something that would make just about anyone else at least smile. If there was anyone else close enough to hear what I said, they'd usually laugh, but not her.
Finally after well over a year, she cracked a smile once. I suspect that she must have been having the best day she had in her adult life for that to happen.
I decided that it took far too much effort to get her to smile and I would never get her to laugh. So I never tried to make her laugh again.
She passed away about a year ago. As far as I know, that one time I got her to smile may have been the only time she smiled in at least twenty years.