It is natural to all men that they love one another. A sign of this is that, by a certain natural instinct, one man helps another man, even a stranger, in necessity. For example, to warn them when they are going the wrong way, to rescue them if there is an accident, and so forth, as if every man was an acquaintance and friend of every other man. Therefore, mutual love is commanded to men by divine law. A civil community, such as the state, is bound by a sort of friendship, but for the state to achieve its end, which is the good of its members, something more is needed than the mere bond of friendship.
Civil authority is needed.
Si ergo naturale est homini quod in societate multorum vivat, necesse est in hominibus esse per quod multitudo regatur. If therefore it is natural for man to live in a society of many, it is necessary that there be something among men by which the multitude is governed. Diversorum autem diversae sunt causae. Diverse things have diverse causes. From the above, we can see that civil authority would still be needed even if all the members of society were perfect and unselfish.