Speaking personally, depression is more about the ability to manage emotions rather than the emotion themselves. In other words, remembering something that happened in the past that would cause a tiny bit of embarrassment in a "normal" condition would be unbearably painful under depression.

Therapy is useful both in recognizing when your emotional response is disproportional compared to the cause (that is, being "properly" sad or embarrassed vs being depressed), snap out of it and to "learn" to avoid being embarrassed or sad for things outside of your control that could trigger your depression, for example. Something along the same lines of the "there's no spoon" quote from the matrix, but applied to your emotions.

Speaking from my own personal experience, telling myself that the specific facts at hand were irrelevant was very tempting and totally wrong. It had everything to do, for me, with the details of the emotion and the situation, and critically what it was linked to in my history. I suspect that’s true for everyone, but I couldn’t come close to proving it.

>If the issue were biological lack of electricity, why would childhood trauma be linked to it? And why would it be curable through therapy?

Regardless if the serotonine theory of depression is real or not, something having a biological substrate doesn't mean its hardwired or only kickstarted by biological factors.

Levels of hormones, neurotransmitters, and other substances can drop due to many factors, including diet, exercize, sun exposure, vitamin uptake, issues with insulin, thyroid, and so on, but also "psychological" things like trauma response, stress, continued fight or flight mode, etc.