Choose To Be Grateful. It Will Make You Happier – Arthur Brooks, New York Times November 22, 2015.

“Beyond rotten circumstances, some people are just naturally more grateful than others. A 2014 article in the journal Social Cognitive and Affective Neuroscience identified a variation in a gene (CD38) associated with gratitude. Some people simply have a heightened genetic tendency to experience, in the researchers’ words, “global relationship satisfaction, perceived partner responsiveness and positive emotions (particularly love).” That is, those relentlessly positive people you know who seem grateful all the time may simply be mutants…”