Doesn't your own brain create serotonin?

Yes. The locus ceruleus in the brainstem is the chief source of serotonin but other brainstem areas may contribute. Depression is considered to be primarily a low serotonin syndrome. There are other conditions associated with low serotonin function. Also low serotonin is not the only cause of depression.
Raphe Nuclei. A respectful correction: raphe nuclei produce serotonin in your own brain. Locus coeruleus produces norepinephrine as well. They are closely located in the brainstem. Both systems can be boosted by blocking the re-uptake of either chemical to help depression by meds such as effecxor or Cymbalta (duloxetine) (snri = serotonin norephinephrine re-uptake inhibitors).