I do the same, even when you are depressed you still laugh at funny jokes and stuff being depressed is not having a sad face all the time, but much of the times i seem happy i do it on purpose its a mask i wear to hide what i feel to everyone, the only ones that know how i feel are the ones i spoke about it and even they can't fully understand my feelings.

Anyway its normal to laugh at jokes or smile when you see a friend someone you like it also happens to me i also don't feel anything inside so i don't know if that is a normal reaction after spending too much time wearing a mask to keep my feelings hidden or if its just simply normal.