Arguably it's life experiences and environmental stimuli that actually play a role in depression et al - it's just these neurotransmitters you speak of are the communicators. E.g. Self-protective mechanism from trauma (physical and/or emotional) to "close ones heart" or develop defensive behaviours - and then being locked into that state, indoctrinated into it - without the ability to disconnect from that pattern until say dramatically changing the chemical supply of transmitters - say either MDMA flooding the brain with serotonin, more than the brain naturally can at once, or say through ketamine or Ayahuasca or other psychedelics that give the opportunity for a perception change outside of the indoctrinated state(s) and behaviours.

I imagine the vast majority of people who's circumstances aren't due to a chemical issue, more so the lack of adequate support for them and/or any element of Maslow's Hierarchy of Needs not being met during rapid and crucial childhood development.

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.