Why rousing grace is needed: supernatural acts exceed the will's native power
Thesis XXXVIII. They point out that the need for this grace arises because supernatural acts, such as those that make up a person's conversion, far exceed the will's inborn and native power.
So supernatural strength and a special grace that goes before the act of the will itself are needed to produce such an act, which is not the case for the will's other, natural acts that are proportioned to its power.
Latin
XXXVIII. Hujus autem gratiae necessitatem monent ex eo oriri, quod actus supernaturales, quales sunt illi in quibus consistit hominis conversio, congenitam & ingenitam voluntatis potentiam longe excedunt, ideoque viribus supernaturalibus, & speciali gratia, antecedente ipsum voluntatis actum, ad hunc eliciendum opus est; quod ad alios actus naturales voluntatis ejus potentiae proportionatos opus non est.