Well, it certainly isn't true that we are always in charge of our moods. (No one is, My Lord.
) And sometimes this is exceptionally true due to simple imbalances in brain chemistry. And I'm not a believer that these imbalances should always lead to medical treatment, but it's a simple biological fact. The brain, as complex as it is, is just another organ, after all. Very often, just changes in diet, exercise and environment can help to improve them (as snowy suggested).
It should also be noted that there is a lot of evidence that mood disorders can also be caused by
allergies.