The grass is always greener on the other side. I'm sure more solitary individuals will regret not being more socially active and having fun. On the other side though, you will also definitely regret partying too much if it impacts your life outside of it. I would guess that moderation is key and not being too far on one side or the other could optimize for happiness (but it's a guess.. I was too far on the partying side of the spectrum for a good portion of my life and then I over corrected to the other end.. so there are regrets on both ends).