http://www.huffingtonpost.com/2013/12/09/scientific-proof-that-you_n_4384433.html
One theory in psychology research suggests that we all have a happiness “set-point” that largely determines our overall well-being. We oscillate around this set point, becoming happier when something positive happens or the opposite, afterwards returning to equilibrium.
But this set-point, to a certain extent, can be reset. Although our general mood levels and well-being are partially determined by factors like genetics and upbringing, roughly 40 percent of our happiness is within our control, according to some experts, and a large body of research in the field of positive psychology has shown that happiness is a choice that anyone can make. As psychologist William James put it, “The greatest discovery of any generation is that a human can alter his life by altering his attitude.”