Caught laughing in a hospital

You don't laugh in a hospital because people are sick and not really in a happy mood, so, seeing you laughing might aggravate them. I think, the idea behind this is that patients in a hospital want to be at peace and they are not looking for any giggles or jokes you usually tend to crack. Fun is injurious to health in a hospital emergency ward. 

Think about it for a while, say, you took a friend to a hospital and it seems everything is fine with her, and that news helps you relax and you are back in your office conversation mood...would you just start being in a jolly mood, do you not realize that there are a lot of others around you who did not get to hear about their friends the way you got to hear about your friend. So, doesn't it makes sense, not to express your usual jolly self in an emergency ward. I know, it's common sense and pretty basic, but, if you are always happy and you hear no bad news in a hospital then it's really difficult to stop yourself from being in a jolly mood. 

Hospital is not like a library, the later keeps a "Keep silence" board, but the former doesn't have a board that says "Don't laugh/giggle" (it's just understood).