Trying to understand what makes us happy is a very interesting field of modern research. In the TedX video below, Matt Killingsworth explains the link between mind-wandering and happiness that his research has uncovered.
Using a mobile app he was able to generate 650,000 sets of data responses from people reporting on their level of happiness.
His findings indicated a direct relationship between a wandering-mind and its negative impact upon happiness. He also discovered that our minds wander a lot, so this relationship is difficult to prevent.
The reason is that when people let their minds wander, they tend to think more about negative things than positive or neutral things. As an example, people might start thinking about an argument they had the previous day with their spouse and then start to stress about that conversation, therefore making themself unhappy.
It seems that Matt's research highlights the importance of staying in the moment and being focused on whatever we are doing (which statistically keeps you happier than if you let your mind wander) and/or to be far more mindful about our mind-wandering. In other words, consciously choose to let your mind wander about pleasant things.
The video goes for just over 10 minutes and is worth a viewing.