As the name suggests, cognitive, is related to the mind, and behavior is the outside behavior seen in a person.
When our behavior is determined by our thoughts, the thoughts which are negative lead to maladaptive behavior which hampers our day to day living, leading to a variety of problems like anxiety, depression, stress, anger etc.
The overall purpose of Cognitive Behavior therapy is to increase self awareness, facilitate better self understanding and improve self control by identifying the dysfunctional thoughts and beliefs that are predominantly negative, biased and self critical.
It is a process of self monitoring which results in the negative thoughts and beliefs being replaced by positive, balanced and functional thoughts which acknowledge the strengths and success.