Justification consists in God's forgiveness of our sins. What does this forgiveness consist in? At least partly in the taking away of the penalty. But what, most deeply, is the penalty? One thinks here of hell-fire. But while hell may contain fire (or it may contain great cold!), it is not constituted by fire, but by separation from God. Now, lack of charity—lack of the right kind of love for God—is at the heart of separation from God.
So: Divine forgiveness must consist, at least in part, in the removal of the penalty of separation from God, and the removal of our lack of charity. Therefore, the instilling of charity is at least partly constitutive of divine forgiveness. Hence, basic sanctification—the movement from lack of charity to the presence of charity—is not merely causally tied to justification, but is at least partly constitutive of justification. Moreover, this sanctification is not appropriate, and maybe not possible, apart from justification, since a just being is unlikely to waive punishment without forgiving.