I came across this while reading Harry Potter. Here I attached the paragraph I've taken this from.
Moody’s magical eye spun around to stare at Ron; Ron looked extremely apprehensive, but after a moment Moody smiled — the first time Harry had seen him do so. The effect was to make his heavily scarred face look more twisted and contorted than ever, but it was nevertheless good to know that he ever did anything as friendly as smile. Ron looked deeply relieved.
It seems to me that harry feels good to know that Moody in his life apart from all the unfriendly things had done something friendly i.e, the smile.