The SSRIs cause more circulating seratonin, which means there is less seratonin waiting to be released by the action of the LSD. Your high will be diminished, but also will last longer.

You're probably thinking of MDMA, which releases serotonin. LSD doesn't release seratonin, but it does have similar shape to serotonin and therefore can trigger the same receptors that serotonin does.

That's brain-chemistry-related, with psychedelics competing with the SSRI for your serotonin receptors.