They're biological. A lot can happen :) Remember this includes weirdness like viruses, tumors, aging... as well as specifically neural stuff like habituation (d

They're biological. A lot can happen :) Remember this includes weirdness like viruses, tumors, aging... as well as specifically neural stuff like habituation (diminished sensitivity to repeated stimulation), sensitisation (increased sensitivity to repeated stimulation), spontaneous misfiring, reorganisation (basically creating/destroying or upweighting/downweighting synapses) etc etc etc. Of course, then you'll need to debug problems by dripping antidepressants (or psychedelics, or stimulants, or depressants, or various hormones, or....) into the Petri dish. Biology is wonderfully crazy.