Well first let's look at which chemicals are imbalanced in those suffering from clinical depression. Currently, most research being done focuses on serotonin levels.

I'm sure everyone here has heard that low serotonin levels are believed to be associated with depression. Another cause of depression being researched is the process of neurogenesis in the hippocampus. In a clinically depressed patient, neurogenesis is slower and therefore makes a weak hippocampus (the store for long term memory, also associated with one's mood).

Continuing to look at the biochemical aspects of depression, let's move onto how they connect to social and environmental factors, the other potential cause mentioned in the original question. Sure, we all accept that if one's relative dies, one goes through a divorce, or one experiences a traumatic accident or something, they are more likely to suffer from depression than is an unscathed individual. Why? This could also be a biologically based issue.

Let's say you're in a traumatic car accident. Everyone dies except you, maybe you were badly burned and it was a close call getting out of the car even for you. Every night you have nightmares about the accident and as a result, you eventually lose sleep. Loss of, or broken sleep, is a known symptom of depression. I'm just using this as an example because it comes from a chemical situation.

Whether or not a traumatic event, or difficult time in life can easily bring you down to depression, that could easily be an issue of genetic predisposition. In the human genome, we have an innumerable amount of genes responsible for every different quality of our existence on the exterior and interior. The neurons in my body, my hippocampus, my brain overall, they're different than yours. And yours are different than the next person. So, from a physiological standpoint, these structural differences could also contribute to how easily one can end up depressed.

So, sure, I doubt someone can become depressed just because of the complex ideas of genetic predisposition we just discussed, but it can facilitate such a response to a given stimulus. The stimuli I speak of, are the environmental factors. Everyone goes through life in a different manner. We all live in different places, work different jobs, have different stressors and methods of relaxation. We all grew up in different households and faced different issues. So each individual has a different reason, and a different degree to which their "button" has been pushed to start them toward depression. The signaling mechanisms of the nervous system change in response to these environmental factors. So, the chemical imbalance that is predisposed to a different degree in individuals, can be triggered by life events too. Predisposition for thiese chemical reactions is silent until an even of X magnitude happens, X being the magnitude required to make a person depressed. X is different for everyone, and it is unknown.

Finally, what I'm saying is that both factors are a team. To answer which one came first, it's neither. The chemical imbalance didn't come first, and neither did the environmental factors. The predisposition for the chemical imbalancing response to potential environmental factors. is what came first. Then one's existence started, and it is up to fate whether or not some event passed the threshold that would make an individual person depressed. But this can't be generalized, which is why I say that environmental factors didn't come first.

I'd be interested to know what everyone else thinks, I don't really have a straight answer to this and I don't think one exists. A genetic chemical imbalance gives a person the likelihood to react to the environmental factors, but unless the environmental factors are presented, then it's fairly impossible to know if the genes are present and active. So, I bring you right back to the same thing. Which came first, the chicken or the egg? It's impossible to know in the case of depression because without the egg we don't know the chicken exists because it's not the size of a chicken, and without the chicken we don't know how the egg came to exist.