I'm sorry to say that there is not a lot of good science to support the idea that these conditions are caused by chemical imbalances. Anger, happiness, and sadness have not been studied because they are accepted as normal and reasonable human emotions; however, depression has been extensively studied and given disease status.
The common concept that low levels of serotonin in the brain cause depression has its roots in the 1960's. At that time it was discovered that depressed people had measurably lower levels of serotonin than non-depressed people, and pharmaceutical companies promptly developed and began to market SSRI's, or Selective Serotonin Reuptake Inhibitors, such as Prozac. These drugs are designed to increase the level of the neurotransmitter serotonin in the patient's brain.
The problem is that we have a chicken-or-the-egg problem here. More recent research indicates that depression, which has a number of physical effects, creates the lowered serotonin levels, and not the other way around. However the pharmaceutical companies are in the business of selling drugs, and they have worked hard to convince both the public and the medical community that depression is simply a chemical imbalance, and is easily treatable with drugs.
The first link below will take you to a pdf of an excellent scientific review of the literature on this topic.