"sloth is essentially a spiritual vice with a spiritual object. For what the slothful person resists is the whole 'new self' (not just the soul), that is, becoming a person transformed by the Spirit of Christ. She resists it on grounds of her residual attachment to the whole 'old self' (not just the body), that is, all her habits and desires that are rooted in rebellion toward God and prideful attempts at autonomy.
"Conflict between the old and new is possible because the transformation between being sinful and becoming redeemed is not instantaneous. This is equally key to understanding sloth as a vice. The infusion of charity orients the will to its supernatural telos and enables the will to reach that end. But this requires a long process of rehabituation and transformation, a process that requires human cooperation and consent. Infused virtue does not automatically or initially make action in compliance with charity pleasant or easy. Rather, this long, arduous process involves the discipline of dying to the old self and resisting its inclinations. There is much effort involved in this daily commitment to transformation. Love takes work.
"this ongoing transformation—with the struggle it often brings to the surface between old and new habits and desires—is the object of the slothful person's resistance and aversion. So Aquinas summarizes his definition of sloth by saying, 'And so when desire of the flesh is dominant in human beings, they have distaste for [this interior divine] spiritual good as contrary to themselves [sibi contrarium].'
"Hence there is aversion to effort and a desire to remain comfortable and undisturbed involved in the vice of sloth. The effort shunned and the comfort sought, however, are not most accurately described merely in bodily, physical terms. . . . one of sloth's main symptoms is investing great effort in diversionary activities, activities that might well require much physical effort. By remaining preoccupied with these activities, the slothful one avoids accepting the demands made by divine love, demands directed toward the regeneration of one's nature.
"This explains why restlessness (false activity) and intertia (false rest)—are the twin marks of a slothful character. When the apparent evil feels escapable, a person will go to almost any length to avoid facing up to her identity as a friend of God and the attendant demands of this relationship. This avoidance can show itself in restless activity, diversion-seeking, and even physical restlessness. . . .
"On the other extreme, when the apparent evil of our participation in the divine nature feels inescapable, sloth shows itself as inertia, oppressive sorrow, and even despair. The slothful person cannot bear to give up on happiness but she also cannot bear to endure what true happiness requires of her. . . ."
Rebecca Konyndyk DeYoung, "Aquinas on the vice of sloth: three interpretative issues," The Thomist 75 (2011): 56-58 (43-64). Sloth is "'the dislike, horror, and detestation of the Divine good'" (ST II-II.35.a3).