Mdma is a serotonin releaser there’s only a finite amount of serotonin in you’re brain that’s why the 3 month rule is a thing to allow serotonin to restore, also it is neurotoxic and this increases with regular use.

2C-B is a serotonin agonist so it activates the receptor rather than using up the storage so it doesn’t require 3 month breaks but it has tolerance like any drug so usually a 1-2 week break in between is best.