Behaviorism began as a form of research psychology. For the early part of the 20th century, the behaviorists were strictly scientists, working in universities on psychological research problems. They did not really start to think about mental illness and psychotherapy until the 1930s and 40s, well after the psychodynamic therapists had all but cornered the market.
To the behaviorist, human behavior has nothing to do with internal unconscious conflicts, repression, or problems with object representations. Rather, a behavioral psychologist uses principles of learning theory to explain human behavior. According to behavioral theory, dysfunctional or unhelpful behavior such as depression is learned. Because depression is learned, behavioral psychologists suggest that it can also be unlearned.
In the mid 1970s, Peter Lewinsohn argued that depression is caused by a combination of stressors in a person's environment and a lack of personal skills. More specifically, the environmental stressors cause a person to receive a low rate of positive reinforcement. Positive reinforcement occurs when people do something they find pleasurable and rewarding. According to learning theory, receiving positive reinforcement increases the chances that people will repeat the sorts of actions they have taken that led them to receive that reinforcement. In other words, people will tend to repeat those behaviors that get reinforced. For example, many people show up at work on a regular basis in order to receive money or insurance benefits. Most academically-minded children study in order to help ensure that they will continue to receive good grades. In these examples, working and studying are behaviors that are motivated by money, benefits, and good grades, which are positive reinforcers.
According to Lewinsohn, depressed people are precisely those people who do not know how to cope with the fact that they are no longer receiving positive reinforcements like they were before. For example, a child who has newly moved to a new home and has consequently lost touch with prior friends might not have the social skills necessary to easily make new friends and could become depressed. Similarly, a man who has been fired from his job and encounters difficulty finding a new job might become depressed.
In addition, depressed people typically have a heightened state of self-awareness about their lack of coping skills that often leads them to self-criticize and withdraw from other people (e.g., depressed people may avoid social functions and get even less positive reinforcement than before). To make matters worse, some depressed people become positively reinforced for acting depressed when family members and social networks take pity on them and provide them with special support because they are "sick". For example, some spouses may take pity on their depressed partners and start to do their chores for them, while the depressed person lays in bed. If the depressed person was not thrilled to be doing those chores in the first place, remaining depressed so as to avoid having to do those chores might start to seem rewarding. Research suggests that Lewinsohn's theory explains the development of depression for some individuals, but not for all.
Traditionally, behaviorists did not pay much attention to people's thoughts, perceptions, evaluations or expectations and instead focused solely on their external and directly observable and measurable behavior. They did this not because they weren't aware of these internal feelings and thoughts, but because they thought them relatively irrelevant to the process of influencing behavior, and too difficult to measure with any accuracy. It turns out that this position was too extreme. More recently, research has shown that internal events such as perceptions, expectations, values, attitudes, personal evaluations of self and others, fears, desires, etc. do affect behavior, and are important to take into account when doing therapy. As a result, old-fashioned "strict" behavioral approaches to treating depression are not as popular today as they used to be.