Objection 1. It would seem that a habit cannot diminish. Because a habit is a simple quality and form. Now a simple thing is possessed either wholly or not at all. Therefore although a habit can be lost it cannot diminish.
Objection 2. Further, if a thing is befitting an accident, this is by reason either of the accident or of its subject. Now a habit does not become more or less intense by reason of itself; else it would follow that a species might be predicated of its individuals more or less. And if it can become less intense as to its participation by its subject, it would follow that something is accidental to a habit, proper thereto and not common to the habit and its subject. Now whenever a form has something proper to it besides its subject, that form can be separate, as stated in De Anima i, text. 13. Hence it follows that a habit is a separable form; which is impossible.
Objection 3. Further, the very notion and nature of a habit as of any accident, is inherence in a subject: wherefore any accident is defined with reference to its subject. Therefore if a habit does not become more or less intense in itself, neither can it in its inherence in its subject: and consequently it will be nowise less intense.
On the contrary, It is natural for contraries to be applicable to the same thing. Now increase and decrease are contraries. Since therefore a habit can increase, it seems that it can also diminish.
I answer that, Habits diminish, just as they increase, in two ways, as we have already explained (52, 1). And since they increase through the same cause as that which engenders them, so too they diminish by the same cause as that which corrupts them: since the diminishing of a habit is the road which leads to its corruption, even as, on the other hand, the engendering of a habit is a foundation of its increase.
Reply to Objection 1. A habit, considered in itself, is a simple form. It is not thus that it is subject to decrease; but according to the different ways in which its subject participates in it. This is due to the fact that the subject's potentiality is indeterminate, through its being able to participate a form in various ways, or to extend to a greater or a smaller number of things.
Reply to Objection 2. This argument would hold, if the essence itself of a habit were nowise subject to decrease. This we do not say; but that a certain decrease in the essence of a habit has its origin, not in the habit, but in its subject.
Reply to Objection 3. No matter how we take an accident, its very notion implies dependence on a subject, but in different ways. For if we take an accident in the abstract, it implies relation to a subject, which relation begins in the accident and terminates in the subject: for "whiteness is that whereby a thing is white." Accordingly in defining an accident in the abstract, we do not put the subject as though it were the first part of the definition, viz. the genus; but we give it the second place, which is that of the difference; thus we say that "simitas" is "a curvature of the nose." But if we take accidents in the concrete, the relation begins in the subject and terminates in the concrete, the relation begins in the subject and terminates at the accident: for "a white thing" is "something that has whiteness." Accordingly in defining this kind of accident, we place the subject as the genus, which is the first part of a definition; for we say that a "simum" is a "snub-nose." Accordingly whatever is befitting an accident on the part of the subject, but is not of the very essence of the accident, is ascribed to that accident, not in the abstract, but in the concrete. Such are increase and decrease in certain accidents: wherefore to be more or less white is not ascribed to whiteness but to a white thing. The same applies to habits and other qualities; save that certain habits and other qualities; save that certain habits increase or diminish by a kind of addition, as we have already clearly explained (52, 2).
Source: Summa Theologica (New Advent)