I always find it tempting to draw comparisons between stuff that goes on in the brain with things we find more familiar. Those of us who live in the northern hemisphere, for example, will have noticed the weather is starting to turn chilly. Last night we had frost and so our thermostat was turned up to compensate. The limbic system is a part of the brain that helps to control mood and it has been likened to a thermostat in the way it maintains our mood. It's a useful comparison in that, like any piece of equipment, the limbic system can malfunction.
If you're happy for yet more comparisons let me introduce you to the notion of the reverberating circuit (skip this paragraph if you're an electrician). You'll find these circuits in a variety complex machines but I'll use an aircraft as my example. Imagine that at some point into your flight a strong crosswind slows your progress, so the autopilot compensates by increasing the throttle. As a result, more fuel will be used and extra calculations then take place to see if this can be done. While this is going on the wind is pushing the aircraft off course, so the autopilot adjusts the rudder. In this example the effect of the environment (the wind) triggers a series of inputs and outputs by the circuit in order to keep things running smoothly, on course and on time.
Thanks in part to our limbic system our moods are pretty stable. If I suddenly became very wealthy there's a very good chance my mood would peak. However, I know my mood will eventually return to what it was before my riches and this tells me the overall control of my mood is internal, but can be affected by external events.
Things like peak moments, alcohol, viral infections, drugs or hormonal conditions all affect the limbic system but the biggest culprit of the lot is stress. Like the thermostat or the reverberating circuit it can malfunction and when this happens the characteristic set of symptoms that we use to define depression appear.
There's a limit to the comparisons we can draw and the biggest limit with an issue like depression is the fact that we don't really know why it occurs, or why it differs in terms of it's intensity or duration. Check out a malfunctioning circuit board and you might find the cause and be able to replace the part. This isn't the case with the brain. We appear to understand more than ever about depression but the relationship between hormones and neurotransmitters in the brain is complex and not fully understood at a biological level. Add this to our lack of knowledge about how the things in life affect us at a neurochemical level and it's easy to see why our real understanding of depression remains so sketchy.