Unlike other animals, human beings spend a lot of time thinking about what is not going on around them, contemplating events that happened in the past, might happen in the future, or will never happen at all. Indeed, “stimulus-independent thought” or “mind wandering” appears to be the brain’s default mode of operation… Our mental lives are pervaded, to a remarkable degree, by the nonpresent. A human mind is a wandering mind, and a wandering mind is an unhappy mind. Mind-wandering is an excellent predictor of people’s happiness. In fact, how often our minds leave the present and where they tend to go is a better predictor of our happiness than the activities in which we are engaged…. [So] ..The ability to think about what is not happening is a cognitive achievement that comes at an emotional cost.
Matthew A. Killingsworth and Daniel T. Gilbert, “A Wandering Mind Is an Unhappy Mind”, Science Magazine