I posted above that we had a close relative die tragically. Some close family members laughed and

[anneboleyn]

chatted away at the wake and after the service (they also talked freely to the many hundreds who attended as the relative was a very well-known and beloved person in the community). Observers who didn't know the family would probably say they didn't seem "sad enough." However they have been in severe depressions for years since the death. They have in fact never recovered.

I have seen people giggle when given a terminal cancer diagnosis. They certainly were/are not "happy." As you said it is a stress response.