As stated in the previous post, it was Aquinas' differentiation between acquired moral virtue and divinely infused moral virtue that turned his attention to theological virtue.
According to Thomas, the moral and intellectual virtues are essential for human happiness, but they are insufficient for the attainment of perfect happiness that consists of the beatific vision of God. This kind of happiness cannot be humanly achieved, but requires divine intervention. The former happiness that results from the moral and intellectual virtues is in keeping with divine purposes but are only according to human nature. The latter happiness surpasses nature moving into the realm of the divine. This beatific vision is possible because of the theological virtues which have God himself as their telos, their end, and not just the things that lead human beings toward God. It is not only this that makes certain virtues theological, they are also theological in that they find their source in God.
The theological virtues supply the desire and motivation to seek their ends. The will and the mind in and of themselves lack the desire to seek God. Thus even the love we have for God himself finds its source in God. What is critical here in reference to the moral and intellectual virtues, is that Aquinas argues that when God infuses the theological virtues into an individual, he infuses all of the moral and intellectual virtues as well.
In his discussion of the intellectual and moral virtues, Thomas, utilizing Aristotle, suggests that the virtues provide a means between two extremes. For example the intellectual virtue of understanding is the mean between ignorance and being a know-it-all. Courage keeps one from the extremes of cowardice and reckless behavior. Self-control keeps one away from indulgence on one hand and extreme asceticism on the other. But such does not appear the case with the theological virtues. Since God himself is the object of these virtues, they are not means between extremes, but rather the more they are employed to the extreme, the more divine happiness can be realized. In other words, it is impossible to believe too much in God, to love God too intensely, and to hope in God beyond all reason.
Of course, in order to understand why these theological virtues differ in their "extreme" practice from the intellectual and moral virtues, Aquinas must delineate what he means by the three theological virtues of faith, hope, and charity.