Serotonin has little relevance to depression and was more of a cute story doctors told patients in the '90s, that they were told by pharma reps.

AFAIK the truth is a little more complex: SSRIs do work for many people, but we don't really understand why and it's not as simple as 'more serotonin good.' They also don't work for everyone, and we don't understand why they don't work for some people either.