Changes in Thinking - You may experience problems with concentration and decision making. Some people report difficulty with short term memory, forgetting things all the time. Negative thoughts and thinking are characteristic of depression. Some people have self-destructive thoughts during a more serious depression. Changes in Feelings - You may feel sad for no reason at all.
Some people report that they no longer enjoy activities that they once found pleasurable.
You might lack motivation, and become more apathetic. Sometimes irritability is a problem, and you may have more difficulty controlling your temper. In the extreme, depression is characterized by feelings of helplessness and hopelessness. Changes in Behavior - Changes in behavior during depression are reflective of the negative emotions being experienced. You might act more apathetic, because that's how you feel.