Thoughts are mental behaviors typically taking the form of unspoken conversations, running commentaries, visual images or sounds. They are continuous and constant; always occurring in one form or another throughout our lives. Sometimes we are aware of thoughts, and sometimes we are unaware. We can control them when we choose, but we cannot turn them off. When we are not controlling thoughts ourselves, they are controlling us. Thoughts exert a profound influence on how we feel and what we do. Because thoughts are a type of behavior (albeit a hidden or covert sort of behavior), they can be manipulated by using methods derived from learning theory.
According to the cognitive behavioral model (a highly regarded approach to psychotherapy derived from learning theory), thoughts are like a lens through which you examine and make sense of the world. Events that happen have no inherent meaning. Instead, you assign meaning to the events you experience through a process known as appraisal (which is a fancy word for judgment). In essence, when something happens, you think about that thing, and then figure out what that thing means for you and your life; whether it is good or bad. Your judgments end up having rewarding or punishing properties, depending on whether you think things are good or bad, respectively. The thing that gets punished or rewarded most primarily is your mood. Rewarding thoughts (positive appraisals) enhance your mood and punishing thoughts (negative appraisals) tend to depress it or make you anxious.
Though events have no inherent meaning, there is such a thing as shared social consensus. A group of people can think about an event and all pretty much come up with a consensus about whether that event is good or bad, and also how good or how bad. For example, most people think that a death in the family is a pretty bad thing, and also that having to stand in line for a long time is annoying, but ultimately not so bad. This sort of shared social consensus is about as close as it is possible to come to knowing what an event "should" mean.
So long as your own appraisals are pretty much in line with shared social consensus (e.g., so long as you see events pretty much as other people around you see them), we can say that your appraisals are pretty much accurate and balanced. You are likely, for instance, to get mildly annoyed when you find yourself in a long line, and unlikely to throw a fit and scream and yell about it. The former response would be in proportion to the magnitude of the problem, while the latter would be an over-reaction.
Not everyone's appraisals are in line with shared social consensus, however. Some people develop appraisal habits or biases, and the presence of these habits color or bias the way that these people make appraisals. Two popular ingrained thinking biases are known as optimism and pessimism Optimists are "glass half full" people; they are biased to see the world as a more positive place than it really is. Pessimists are "glass half empty" people who are biased to see things more negatively than they are. The proverbial glass is actually neither half full or empty, (neither cause for celebration or for despair), but instead, simply takes on the meaning assigned to it by the person doing the judging.
Most typically, people seeking self-help knowledge have developed pessimistic habits of seeing things as more negative than they really are. Their negative appraisal habit leads them to over-react in a negative direction. They become intensely worried, anxious, guilty, ashamed, sad or depressed in a manner that seems quite out of proportion to someone who does not share their negative appraisal biases. Because the appraisal bias is chronic and habitual, the out of proportion mood reaction also becomes chronic. Over time, the faulty or dysfunctional appraisal habit can lead to chronic mood and self-esteem problems and disorders which negatively affects people at all levels.
In summary, the cognitive behavioral argument suggests that problematic and biased appraisal habits lead people to see the world in an overly negative manner, causing them to feel disproportionately depressed, anxious, angry or self-pitying. These moods and the disorders that develop around them can be alleviated in part or in full through the use of cognitive techniques designed to help people learn new and more accurate ways of appraising life events.