Your friends serotonin theory REALLY makes sense. I am learning a lot about this cleanse business and the toxins from parasites - much in only three weeks from this site - but neurotransmitters, I know a lot about - and the toxic levels (consider the gunk given off by parasties and the ethanol given off by Candida. . .) would certainly adversely affect serotonin! the toxins interfere right at the synapse, so says my MD friend - causing it all to be used up too quickly, for us to use properly - and also can cause too little S to be released in the first place - which affects domapine and other neurotransmitters!
I think you and your friend are on to something applicable, perhaps, to many people here.