It is also worth mentioning that Aquinas saw the tendency toward self-interest as one of the reasons government must exist. Logically, then, the more selfish the population becomes, the more it will need to be parented by the State. The greater the emphasis on self-interest, the greater the need for a superior authority to look after the common good: “For where there are many men together and each one is looking after his own interest, the multitude would be broken up and scattered unless there were also an agency to take care of what appertains to the commonweal.”
In a fully self-interested society, what “agency” is there to look to the commonweal but the totalitarian State? And this is precisely what the Church does not want. The State has a proper sphere, which is larger than some moderns would prefer to admit, but it is forced out of this sphere today by the very individualism that pretends to condemn state-overreach. Through individualism and the doctrine of self-interest, all intermediate groups and associations are torn apart, leaving only the State to pick up the pieces, growing into a behemoth in the process:
“…things have come to such a pass through the evil of what we have termed ‘individualism’ that, following upon the overthrow and near extinction of that rich social life which was once highly developed through associations of various kinds, there remain virtually only individuals and the State.”
 DR, 8.
 QA, 78.