I strongly think that for every depression there is a reason. Or multiple. Doesn't matter really. If you find it and deal with it, it's over. Medication is only treating the symptoms, but not the cause.
Um, no. Several forms of depression are caused by abnormalities in the brain or it's day to day function.
Mine is a neurotransmitter level inbalance caused by irregular re-uptake of serotonin causing the levels to be too high or too low. It was not "caused" by anything, as in it's more likely genetic, not social/environmental, enzyme activity regulating the re-uptake is...broken.