> but like any powerful drug why would I not suspect something that floods your brain with serotonin would not end up in a situation where your brain anticipates the drug and creates an dependency situation where your brain does not produce serotonin in normal levels without it?

Because this is such an incredible oversimplification of what goes on in the brain. SSRIs also dramatically increase the overall serotonin flux, to the point where if you quit suddenly, the brain is left in a deficit, the effects of which can be quite unpleasant. And contrary to the "serotonin is happy chemical, more serotonin = more better" myth, the net result of all this serotonin is that 5-HT receptors actually downregulate. Ultimately, the result of all these changes is an increase in neuroplasticity.

Blasting 5-HT receptors (eg by LSD, psilocin, etc, and to an extent MDMA) is also shown to increase neuroplasticity.

Neurotoxicity and psychopharmacology are extremely complicated. It's beyond reductionist to say "drug floods brain ergo bad".