“A study published in 2010 drives home this point. The scientists found that inner experiences consistently dwarf outer ones. What participants were thinking about turned out to be a better predictor of their happiness than what they were actually doing. This speaks to a sour experience many people have had: You’re in a situation in which you should be happy (spending time with friends, say, or celebrating an accomplishment), but a ruminative thought swallows your mind. Your mood is defined not by what you did but by what you thought about.”