Objection 1. It would seem that contrition should be on account of original sin. For we ought to be contrite on account of actual sin; not by reason of the act, considered as a kind of being, but by reason of its deformity, since the act, regarded in its substance, is a good, and is from God. Now original sin has a deformity, even as actual sin has. Therefore we should be contrite on its account also.
Objection 2. Further, by original sin man has been turned away from God, since in punishment thereof he was to be deprived of seeing God. But every man should be displeased at having been turned away from God. Therefore man should be displeased at original sin; and so he ought to have contrition for it.
On the contrary, The medicine should be proportionate to the disease. Now we contracted original sin without willing to do so. Therefore it is not necessary that we should be cleansed from it by an act of the will, such as contrition is.
I answer that, Contrition is sorrow, as stated above (1, 1,2), respecting and, so to speak, crushing the hardness of the will. Consequently it can regard those sins only which result in us through the hardness of our will. And as original sin was not brought upon us by our own will, but contracted from the origin of our infected nature, it follows that, properly speaking, we cannot have contrition on its account, but only displeasure or sorrow.
Reply to Objection 1. Contrition is for sin, not by reason of the mere substance of the act, because it does not derive the character of evil therefrom; nor again, by reason of its deformity alone, because deformity, of itself, does not include the notion of guilt, and sometimes denotes a punishment. But contrition ought to be on account of sin, as implying deformity resulting from an act of the will; and this does not apply to original sin, so that contrition does not apply to it.
The same Reply avails for the Second Objection, because contrition is due to aversion of the will.
Source: Summa Theologica (New Advent)