Basically - just because adding serotonin fixed the problem doesn't mean that the issue was low serotonin in the first place. Serotonin levels go up within the first day or so of using SSRIs, but they aren't effective for clinical depression for several weeks. (In disorders that actually have lowered serotonin as a documented effect - like PMDD - SSRIs can be taken on an as-needed basis, for only a few days or a week, to eliminate symptoms.) It's hypothesized that when SSRIs do help for depression, they're creating some sort of second-order effect like increasing neurogenesis or lowering inflammation. Newer research on pharmaceutical treatments for depression generally does not focus on the overly simplistic angle of raising or lowering the levels of various neurotransmitters.

I'm really glad that SSRIs worked so well for you! I just really hate this narrative that depression = low serotonin, therefore serotonin drugs = curing the root cause of depression. It's actively harmful to promote it as SSRIs are effective for only a very small population of people, and it obscures the social causes of depression by flattening things into a purely bio-chemical story.