The current medical orthodoxy is that depression is caused by a neurochemical imbalance in the brain, which is why SSRIs have been prescribed as treatment for it for the past few decades. However, research is emerging that depression might have many underlying causes and our model of it might not be wholly correct.
This article challenges the serotonin orthodox theory of depression:
https://www.nature.com/articles/s41380-022-01661-0