A recent article by Daniel Gilbert and Killingsworth is about mind wandering and happiness.
exerpt:
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...this ability is a remarkable evolutionary achievement that allows people to learn, reason, and plan, it may have an emotional cost.
NYT covering this topic here
a graphic:

Any thoughts from a evolutionary view on this? Happiness is the evolutionary proverbial carrot that drives us towards evolutionary goals: food < survival < reproduction
(EDIT) Fearsclave brought up an interesting point that made me think of this article about: "It was a primitive trip with a sophisticated goal: to understand how heavy use of digital devices and other technology changes how we think and behave, and how a retreat into nature might reverse those effects." interesting read.