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.
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.
Have you ever read criticisms of thought? One idea is that thought has gone loco in modern society. I'm sleep deprived at the moment, and wrote this slightly insane facebook status a couple hours ago:
*The extraordinary intelligence of the body is all that is necessary for good living, but we interfere with its natural operation through the medium of thought. The widespread hoax is that these thoughts are inside of us and special to us; in truth they rampage and trump reality at will. Complexity of modern life has allowed thought to become an overbearing roommate in the studio apartment of your mind.
**Repetitive thought forms belief, and belief becomes unassailable. Absent a practice to let go of thought, that is the human condition. Human is now a misnomer, as we would be better classified as elegant bags of opinions.
I'm inclined to think that a lot of this is environmental, a result of those of us living in cities being stuck in an evolutionarily inappropriate environment. I find that this state of mind tends to go away on fishing and hunting or camping trips. Bopping around in natural surroundings looking for food tends to soothe my mind fairly quickly, especially if I know I have more of it to look forwards to.
People's minds typically wander to unhappy things. They don't have to wander to unhappy things, but they do in the average person because the average person doesn't have his life under control.
I'd look into GTD if you want something actionable to do about this. Getting Things Done is a system which allows you to outsource your worries onto paper and never think them twice. For example, how many times have you thought "Oh I have to buy flashlight batteries" when you're not at the store? That's a useless thought, and all it does is create a little unsettled anxiety. It will probably pop into your mind a few more times in the next couple days too, just creating the same amount of worry.
I'd also look up the book "Flow" by Mihaly Csikszentmihalyi which explains the psychological state of satisfaction in which we are immersed and single-tasking our work.
In terms of evolution: thinking about what might happen to us or has happened in the past to us (in other words, "self-consciousness") would be a good adaptation for learning about certain watering holes to avoid or good places to hunt. But it is generally a very uncomfortable feeling. As has been pointed out, we don't feel it when we "flow." Note that this ability also allows us to anticipate death. This is not to say that it is completely useless, although my own argument for why evolution developed this skill is not very good. I'd like to hear other people's ideas on its "purpose," if it has one. This might be a question that religion or philosophy can answer but not science.