Aquinas argues that ultimate human happiness cannot consist in acts of moral virtue. To be ultimate, happiness cannot be directed to a further end. All moral acts are directed to a further end. For example fortitude in times of war is directed to victory and peace. Justice is directed to keeping the peace.
In our virtues, we observe the mean (which is the right) in the passions within us and in things outside us. The moderation of passions or of external things cannot be the ultimate end of our life since passions and external things can be directed to things less than our highest end and highest happiness.
Man is man through reason, and his proper good, which is happiness, must be in accordance with that which is proper to reason. That which reason has in itself is more proper than that which reason effects in other things. The good of moral virtue is a good established by reason in something other than itself, it cannot be the greatest good of man which happiness is. Rather, this good must be a good that is in reason itself.
Happiness is man’s proper good, that is, the good appropriate to the rational being that is man. Man’s ultimate happiness consists in the ultimate contemplation of truth, which is sought for its own sake, not directed to anything further. Man stands in little need of external things in this ultimate contemplation. All other human operations make way for that ultimate end. Moral virtues and prudence defend against disturbances by the passions.
Ultimate happiness consists in contemplation of the most noble intelligible objects. Such happiness consists in wisdom, based on consideration of divine things. The more perfect the object of intellect the more perfect the intellect.
(Summa contra Gentiles)