I'd not picked up this thread until I read Simeon's article in Community Care, which prompted me to respond. It has been suggested that a chemical imbalance in the brain causes depression, but what about turning that around and thinking whether or not depression can cause a chemical imbalance in the brain - if that is the case, amending the chemical imbalance can reduce the incidence of depression but will explain why it's not a cure.
I have been looking into the work of the Human Givens Institute over the last couple of years and I find their analysis of the causes and responses to depression to be very interesting. In a nutshell depression is caused by being in an emotionally highly aroused state (stress, worry, high levels of anxiety), this in turn causes sleep problems: the sleep problems are based on the theory that the purpose of REM (dreaming) sleep time is for the brain to deal with and repair itself from the 'unexpressed emotions' of the day (too much thinking and worrying about problems and not enough dealing with them) - the brain does this by 'completing' the emotionally arousing events of the day which is experienced through dreaming which is done in metaphore (which is why dreams can often have so many features). If there have been too many unexpressed emotions from the previous day the brain will take longer to discharge these emotions through dreaming, leaving little time for the physical renewal time during sleep, resulting in the patient waking feeling tired. It is suggested that the way in which antidepressants actually work is to reduce the amount of time spent dreaming and so restoring the balance between dream and physically recuperative sleep. I can't do the full explanation justice here but another point suggested is that in the most severely depressed person there is an overlap between the normal REM sleep state and times when the patient is apparently awake, when the patient seems to be hallucinating - this makes sense if you think that in the REM state the brain is working in metaphor. Psychotherapy has a negative effect on depression because it causes the patient to think more deeply about their problems, so increasing the amount of time spent in REM sleep, a vicious circle. Continually going over past problems and issues, rather than discharging them, reinforces them in the mind and actually makes them worse.
Anyone can try the dream theory out for themselves quite easily, although it might take a little practice at first. When you recall a dream make a note of the key features, but particularly the emotions behind what was happening in your dream. Then think back over the previous day, particularly for events that were emotionally arousing but where you did not have the chance to express this at the time (this is the most difficult bit because by the very nature of the dream your brain has discharged the emotions of those events) and the odds are you will find that the emotions you felt in your dreams are the same emotions you felt but did not express the previous day. For example, if you felt exposed in your dream (eg, naked) then you might have felt exposed the previous day when you went to visit a difficult client, or had to 'wing it' at an important meeting. The other features of your dream (people, places) will probably link to people you have met or talked about or places you have been or talked about. Dreaming of walking in a strange town might be because you had a conversation about you or someone else selling and moving house to a new location. Dreaming of committing adultery might be because your partner complained that you spend too much time at work / on the computer / down the pub and not enough with him / her / the family but because you know he / she is right you didn't respond. Having supressed the emotion at the time it comes out in a dream.
I was quite skeptical at first but once I started checking my dreams against the previous day's worries and events it really did begin to make sense. It's be nice to hear of others' experience of trying this.