We don’t yell “Quit drowning!” when we see a guy flailing in the dark waters. Then why do we yell “Cheer up!” when we see someone sad?
We can carry this analogy further: You don’t think before diving in your Armani suit, head long into shark bay, to save your dear friend. But if the dude is bigger than you, or even if he is not, he most likely will take you down with him in his death grip. This is why you knock him out if he starts over-powering you. Only then do you proceed to drag him ashore. I present this as an argument to get drunk with your sad friends. Otherwise, they take you down with them.
My real point is: There are many ways to help your friend who is sad - depending on you, your friend, the weather and your sexual-orientation. But repeating “Cheer Up!” is not one of them. And getting angry is worse - like giving death punishment for attempted suicide.
Everyone, including yours truly, is guilty of this. Common sense is not that common, it seems :-)
Playing, watching a funny movie, going out or any such thing that indirectly takes their mind off their sadness helps, I guess.