Symbiotic relationships aren't all they're "cracked" up to be.
Well, let's say you got an alien symbiote in yer brain that makes you super strong and gives you all sorts of smart advice and stuff to help keep you alive so that you can turn around and help keep it alive.
But in all that, there ain't nothing that would automagically stop you from using all that super strength and smarts to go around killing other guys exactly like youself, guys with their own alien symbiotes, 'cause they are the greatest threat to you and your symbiote in the Survival of the Fittest Competition.
And if you are super successful in the Survival of the Fittest Game, and eliminate all the other fittest, your kids will end up having to marry wimpy no-symbiote parasite-ridden idiots, 'cause that's all that will be left to pick from after you got rid of everybody else.
And your poor symbiote won't have anybody to make out with at all.
Stupid ass know-it-all brain worms, serves 'em right ahaha.