This is a basic model of human experience:

Cognitive therapy sessions examine what thoughts you have about your perceptions, reveal how they are leading to negative experiences (like depression), and then proceed to dispute those thoughts:

Positive thinking, on the other hand, seeks to directly remedy your negative experiences. By telling yourself, "Today is the first day of the rest of your life," you can get a boost or high:

Some element of positive thinking is necessary for a healthy mind, but as an all-encompassing method of therapy, it falls short. What happens when the energy from your "carpe diem" runs out? What happens if the negative thoughts are still there, and only masked by positive thinking.