GGOf course the brain, like any other organ in the body, can develop a malfunction over time. You obviously do not understand how the systems work.
Serotonin is the 'Happy' hormone? (not sure it is a hormone but can't think what else to call it.
The happier you are the more serotonin you manufacture in your brain. When there is a distressing event or change in circumstances or even a series of small sadnesses, eg dreich days, lost purse, after effects of flu or other illnesses, less Serotonin is produced, therefore you feel less happy, therefore even less serotonin is produced - a vicious circle indeed. This is reactive depression and if it does not begin to lift as small happinesses - sunny days, problems solved, time spent with DGC - have their effect and more serotonin is produced, then a short term course of anti-Ds will kick start the return to equalibrium.
Clinical depression is a different kettle of fish entirely. The problem is not that Serotonin is not being produced - it is, but it must make an electric powered leap across a small gap to receptors which uptake the Serotonin for further processing in the brain. These receptors can be faulty or simply deteriorate with age. SSRIs (Selective Serotonin Reuptake Inhibitors) are designed to prevent the serotonin which has been produced, being reabsorbed and reprocessed = Reuptake, by keeping it in the gap beside the receptors, creating a serotonin enriched soup such that the receptors are bombarded and more likely to receive the signals. This leads to a greater sense of well being, which in turn leads to greater production of Serotonin and the cycle is reversed, but will surely deteriorate if the SSRIs are stopped.
I am aware that this a laywoman's explanation of what is going on the brain, but it is how it was explained to me by a Clinical Psychologist. I am sure that, if you are interested in the science behind it, you will find the explanation on line.