Also, if there is a behavioral component to depression as well, then it doesn't necessarily mean someone is to blame for their disorder. You don't control the environment you grow up in, which has an enormous impact even on traits that are highly heritable (the whole subject of heritability is very misunderstood anyway).
So basically if I were going to describe depression's cause, I'd say it's a mixture of biochemical reactions, behavioural traits, and environmental stresses that cause it.
to quote: "Now to the BDNF hypothesis. I used the phrase “unknown mechanism” above, and that’s exactly what this work may have cleared up. The authors show that when the TrkB protein forms a dimer in the cell membrane, a binding site for small molecules is formed at the interface. A whole list of known antidepressants (fluoxetine, imipramine, venlafaxine, moclobemide, ketamine, esketamine, and R,R-hydroxynorketamine) bind to this site at about 1 micromolar levels (and can displace each other in binding assays(, while a set of control CNS compounds like chlorpromazine, diphenhydramine, and indeed S,S-hydroxynorketamine do not. It will not be lost on those who’ve done research in the field that the antidepressant compounds listed above have been thought to work through completely different mechanisms. "