Everyone feels sad, excited or angry sometimes. When kids feel too good, too mad, or too sad, they might have a mood disorder. A mood disorder is an illness of the brain. The feelings, thoughts and behaviors of mood disorders can cause problems in a kid’s life. What causes mood disorders in children is up for debate. There are chemicals in the brain that are responsible for positive moods. Mood disorders may be caused by a chemical imbalance in the brain, on its own or along with environmental factors, such as unexpected life events and/or chronic stress.