Not having read other responses, my attempt to answer in my own words: what goes wrong is that there are tons of possible cognitive influences that could be reinforced by rewards for making people smile. E.g. “make things of XYZ type think things are going OK”, “try to promote physical configurations like such-and-such”, “trying to stimulate the reinforcer I observe in my environment”. Most of these decision-influences, when extrapolated to coherent behaviour where those decision-influences drive the course of the behaviour, lead to resource-gathering and not respecting what the informed preferences of humans would be. Then this causes doom because you can better achieve most goals/preferences you could have by having more power and disempowering the humans.
Not having read other responses, my attempt to answer in my own words: what goes wrong is that there are tons of possible cognitive influences that could be reinforced by rewards for making people smile. E.g. “make things of XYZ type think things are going OK”, “try to promote physical configurations like such-and-such”, “trying to stimulate the reinforcer I observe in my environment”. Most of these decision-influences, when extrapolated to coherent behaviour where those decision-influences drive the course of the behaviour, lead to resource-gathering and not respecting what the informed preferences of humans would be. Then this causes doom because you can better achieve most goals/preferences you could have by having more power and disempowering the humans.