"A wandering mind is an unhappy mind," wrote Harvard University psychologists Matthew Killingsworth and Daniel Gilbert in their study on mental presence. They found that people are happiest when their minds are in the present moment
(yet another reason to meditate)—but that they are only in that state about half of each day. Otherwise, the mind travels, often wistfully or anxiously, leading to unhappiness. The three times we're most likely to be "here" and happy: when exercising, in conversation and (especially) having sex.