For many, if not all there IS a chemical imbalance. To believe otherwise is illogical.
Our moods are controlled and created by chemicals, hormones, serotonin, dopamine, adrenaline, oestrogen the list goes on.

Even 'love' is a drug;
Dr. Helen Fisher, an anthropologist and relationship researcher, conducted a series of illuminating studies on the brain chemistry of love. Specifically, she found that the same brain chemicals (that is, massive amounts of dopamine and norepinephrine ) are in play, and many of the same brain pathways and structures are active when we are falling in love and enjoying a cocaine-high.

I think there is some flexibility in the brain so I'm sure therapy can work for some, in as much as positive thinking, repetition etc can encourage more 'feel good' chemicals to be released.