Objection 1. It would seem that there is not a habit in the will. For the habit which is in the intellect is the intelligible species, by means of which the intellect actually understands. But the will does not act by means of species. Therefore the will is not the subject of habit.
Objection 2. Further, no habit is allotted to the active intellect, as there is to the "possible" intellect, because the former is an active power. But the will is above all an active power, because it moves all the powers to their acts, as stated above (Question 9, Article 1). Therefore there is no habit in the will.
Objection 3. Further, in the natural powers there is no habit, because, by reason of their nature, they are determinate to one thing. But the will, by reason of its nature, is ordained to tend to the good which reason directs. Therefore there is no habit in the will.
On the contrary, Justice is a habit. But justice is in the will; for it is "a habit whereby men will and do that which is just" (Ethic. v, 1). Therefore the will is the subject of a habit.
I answer that, Every power which may be variously directed to act, needs a habit whereby it is well disposed to its act. Now since the will is a rational power, it may be variously directed to act. And therefore in the will we must admit the presence of a habit whereby it is well disposed to its act. Moreover, from the very nature of habit, it is clear that it is principally related to the will; inasmuch as habit "is that which one uses when one wills," as stated above (Article 1).
Reply to Objection 1. Even as in the intellect there is a species which is the likeness of the object; so in the will, and in every appetitive power there must be something by which the power is inclined to its object; for the act of the appetitive power is nothing but a certain inclination, as we have said above (6, 4; 22, 2). And therefore in respect of those things to which it is inclined sufficiently by the nature of the power itself, the power needs no quality to incline it. But since it is necessary, for the end of human life, that the appetitive power be inclined to something fixed, to which it is not inclined by the nature of the power, which has a relation to many and various things, therefore it is necessary that, in the will and in the other appetitive powers, there be certain qualities to incline them, and these are called habits.
Reply to Objection 2. The active intellect is active only, and in no way passive. But the will, and every appetitive power, is both mover and moved (De Anima iii, text. 54). And therefore the comparison between them does not hold; for to be susceptible of habit belongs to that which is somehow in potentiality.
Reply to Objection 3. The will from the very nature of the power inclined to the good of the reason. But because this good is varied in many ways, the will needs to be inclined, by means of a habit, to some fixed good of the reason, in order that action may follow more promptly.
Source: Summa Theologica (New Advent)