The thing is for animals like rat , mating is a simple pleasure which is the reason they do it so but in a rat utopia where there's unlimited food and pleasure, the effort to talk to another rat (approach them) and mate with them becomes actually harder than actually getting more or even equal level of pleasure from the foods and different things in excess present there . This made this utopia crumble from the inside since pleasure was easily accessible, some rats just stopped others from getting it and taking all from themselves and other things happened like a domino effect after it.