According to a study, people are thinking of being somewhere else or doing something else other than what they are doing half of the time and that this mind-wandering makes them unhappy.
Apparently an iPhone application would randomly ask people at different times what they were doing and if they were thinking about their activity or something else that was pleasant, neutral, or negative. The application is available at http://www.trackyourhappiness.org. People were found to be happiest when having sex, exercising, or having a conversation. They were least happy while using a computer, resting, or working.
It's interesting to note that those activities where people were happiest were those that require or bring about presence (or being in the present). Sex and exercise get you out of your mind and into the body. A conversation requires you to listen to someone else. Thankfully, we have a practice that helps us cultivate presence in our life - meditation.