Mood disorders found to be are significantly heritable in twin studies so I don’t think one can say the “vast majority” aren’t due to a chemical issue here. It’s also worth noting that BDNF is not a neurotransmitter, and so having a direct connection with thoughts and personality seems unlikely, though there is reason to believe it may be downstream of long term psychological stresses.
Fair enough, however I'd want to review those studies to see what similarities do exist - even if in different households/environments. Studies that aren't taking into account the relative general lack of health of people in many parts of the world needs to be addressed, accounted for, in research as well.
I do understand how some makeups could allow a large amount of people to certainly to be prone to environmental stresses, have a more fragile-sensitive system - and if those factors are allowed to continue say for decades, there will be a hardening.
