The cause is the will's corruption; delight as its one medicine (chapter 9)
Thesis LXX. On this doctor's view, then, what makes that heavenly delight necessary for good acts is nothing other than the corruption and depravity of the human will, which through the first man's sin has been thrown headlong under the rule of lusts. So in every action before grace it is roused by some lustful delight that goes before it and tickles it; by consenting to that delight it sins, and if left to itself it cannot fail to consent.
To cure this evil God sends the delight of the good, that is, of righteousness, as the sick will's one medicine. Because it runs directly against the disease afflicting the will, it wrings from the will the consent to the good that God requires of it. So the will is turned back upward, so that, fastened to the spirit that soothes and delights it and, as it were, hanging from God, it is no longer dragged down into its old slavery to lust. Jansenius develops this at length in chapter 9 of the book already cited several times.
Latin
LXX. Itaque quod coelestem illam delectationem ad actus bonos necessariam reddit, non aliud est, juxta hunc doctorem, quam voluntatis humanae corruptio & depravatio. Quia, scilicet, voluntas per peccatum primi hominis in libidinum imperium praecipitata est. Ex quo fit ut in omnibus actionibus ante gratiam, delectatione quadam libidinosa praeeunte ac titillante provocetur, cui consentiendo peccat, & si sibi relinquatur, non consentire non potest. Cui malo curando immittitur a Deo delectatio boni seu justitiae, tanquam unica aegrotae voluntatis medicina, qua ut directe contraria morbo quo affligitur, extorquetur ejus in bonum, quod ab ea Deus postulat, consensus; & ita voluntas retorquetur sursum, ut blandienti delectantique spiritui affixa, & quasi suspensa ex Deo, non amplius in veterem libidinis servitutem detrahatur. Quae pluribus diducit Jansenius cap. 9. libri saepius jam citati.