If you're suffering from depression, look into Transcranial Magnetic Stimulation (TMS). After over a decade on SSRIs and other meds had failed, it turned my life around in six months. Literally life saving.
The effectiveness is proven (at much better rates than SSRIs), but the exact mechanism is under study.
But... There was a recent study that suggested that many cases of depression are caused by misordered neuron firing, where the emotional center of the brain fires before the "imagine the future" bit finishes firing. Normally, when a healthy brain imagines a future state, the emotional center fires in response to our anticipated feeling. (Imagination: We're going to the movies. Emotions: FUN) But in a depressed brain, the emotional core fires immediately, resulting in the current, crappy mood being applied to every imagined future. (Emotions: Everything is shit. Imagination: We're going to the movies?)
TMS may work as well as it does because one of the things it does is increase neuroplasticity, allowing the brain to correctly order the firing of our emotional response to imagined futures.
Anyway - TMS is right at the edges of our understanding of treating depression, but it really does work for a supermajority of patients.
For me, I went from having literally lost all emotions and being essentially dead (and being willing to die), to feeling... normal. I haven't had a major depressive episode in the two years since. It's been liberating.