I enjoyed this short personal article by Huda Akil 'In Person: The Pursuit of Happiness' where she examines the personal experience of happiness and joy when playing in a sandpit with her granddaughter & she comments on how we know so little, theoretically, about happiness, joy, contentment.  The article isn't comprehensive, but still worthwhile. Akil comments on the understanding that we have of 'negative' emotions, mainly through research into depression, stress and pain - for which we understand the basic neural correlates.  But, she asks, why don't we have a similar understanding of 'positive' emotions? Where are the animal studies? the studies of hormones? of neural circuitry? This science, relating to positive psychology and affective neuroscience, is still in its infancy ('studies have been done' says the Passive Voice) and its researchers are doing their best to isolate factors that many affect these positive emotions.   And, most importantly, Akil says, 'Science will tell.'