Interesting. I noticed that my POIS symptoms are reduced when I am in social situations for more than a few hours (I live by myself, and work behind a computer, so social situations are the exception rather than the rule, unfortunately, though I am seeing someone atm).
I also noticed that relaxing the cranium and jaw (during a 15-minute meditation-like session) often allows me to reach a kind of euphoric state, which also resolves a lot of POIS-related problems.
Perhaps this is related to the effect of laughter you mentioned.