Objection 1. It would seem that rectitude of the will is not necessary for Happiness. For Happiness consists essentially in an operation of the intellect, as stated above (Question 3, Article 4). But rectitude of the will, by reason of which men are said to be clean of heart, is not necessary for the perfect operation of the intellect: for Augustine says (Retract. i, 4) "I do not approve of what I said in a prayer: O God, Who didst will none but the clean of heart to know the truth. For it can be answered that many who are not clean of heart, know many truths." Therefore rectitude of the will is not necessary for Happiness.
Objection 2. Further, what precedes does not depend on what follows. But the operation of the intellect precedes the operation of the will. Therefore Happiness, which is the perfect operation of the intellect, does not depend on rectitude of the will.
Objection 3. Further, that which is ordained to another as its end, is not necessary, when the end is already gained; as a ship, for instance, after arrival in port. But rectitude of will, which is by reason of virtue, is ordained to Happiness as to its end. Therefore, Happiness once obtained, rectitude of the will is no longer necessary.
On the contrary, It is written (Matthew 5:8): "Blessed are the clean of heart; for they shall see God": and (Hebrews 12:14): "Follow peace with all men, and holiness; without which no man shall see God."
I answer that, Rectitude of will is necessary for Happiness both antecedently and concomitantly. Antecedently, because rectitude of the will consists in being duly ordered to the last end. Now the end in comparison to what is ordained to the end is as form compared to matter. Wherefore, just as matter cannot receive a form, unless it be duly disposed thereto, so nothing gains an end, except it be duly ordained thereto. And therefore none can obtain Happiness, without rectitude of the will. Concomitantly, because as stated above (Question 3, Article 8), final Happiness consists in the vision of the Divine Essence, Which is the very essence of goodness. So that the will of him who sees the Essence of God, of necessity, loves, whatever he loves, in subordination to God; just as the will of him who sees not God's Essence, of necessity, loves whatever he loves, under the common notion of good which he knows. And this is precisely what makes the will right. Wherefore it is evident that Happiness cannot be without a right will.
Reply to Objection 2. Every act of the will is preceded by an act of the intellect: but a certain act of the will precedes a certain act of the intellect. For the will tends to the final act of the intellect which is happiness. And consequently right inclination of the will is required antecedently for happiness, just as the arrow must take a right course in order to strike the target.
Reply to Objection 3. Not everything that is ordained to the end, ceases with the getting of the end: but only that which involves imperfection, such as movement. Hence the instruments of movement are no longer necessary when the end has been gained: but the due order to the end is necessary.
Source: Summa Theologica (New Advent)