Your brain is a complex network that processes vast quantities of information every second. Part of the brain's information-processing network includes neurons, or cells that transmit signals throughout the brain. Neurons send signals through neurotransmitters, which are chemicals some release and others receive. These chemicals essentially let the parts of the brain communicate with each other.
The three most commonly studied neurotransmitters are dopamine, serotonin and norepinephrine. Dopamine is related to experiences of pleasure and the reward-learning process. In other words, when you do something good, you're rewarded with dopamine and gain a pleasurable, happy feeling. This teaches your brain to want to do it again and again. Serotonin is a neurotransmitter associated with memory and learning. Researchers believe it plays a part in the regeneration of brain cells, which has been linked to easing depression. An imbalance in serotonin levels results in an increase in anger, anxiety, depression and panic [source: Nazario]. Norepinephrine helps moderate your mood by controlling stress and anxiety.
Abnormalities in how the brain receives and processes these chemicals can have a big effect on your emotions. For example, when you do something rewarding or pleasurable, the part of your brain that processes that information interacts with the chemical dopamine. If your brain can't receive dopamine normally, the result is that you feel less happy -- or even sad -- after what should have been a happy experience. Studies of people with major depressive disorder (MDD) have shown that they have fewer serotonin receptors in their brains [source: National Institutes of Mental Health].