Objection 1. It would seem that it is unfittingly commanded that man should love God with his whole heart. For the mode of a virtuous act is not a matter of precept, as shown above (1, ad 1; I-II, 100, 09). Now the words "with thy whole heart" signify the mode of the love of God. Therefore it is unfittingly commanded that man should love God with his whole heart.
Objection 2. Further, "A thing is whole and perfect when it lacks nothing" (Phys. iii, 6). If therefore it is a matter of precept that God be loved with the whole heart, whoever does something not pertaining to the love of God, acts counter to the precept, and consequently sins mortally. Now a venial sin does not pertain to the love of God. Therefore a venial sin is a mortal sin, which is absurd.
Objection 3. Further, to love God with one's whole heart belongs to perfection, since according to the Philosopher (Phys. iii, text. 64), "to be whole is to be perfect." But that which belongs to perfection is not a matter of precept, but a matter of counsel. Therefore we ought not to be commanded to love God with our whole heart.
On the contrary, It is written (Deuteronomy 6:5): "Thou shalt love the Lord thy God with thy whole heart."
I answer that, Since precepts are given about acts of virtue, an act is a matter of precept according as it is an act of virtue. Now it is requisite for an act of virtue that not only should it fall on its own matter, but also that it should be endued with its due circumstances, whereby it is adapted to that matter. But God is to be loved as the last end, to which all things are to be referred. Therefore some kind of totality was to be indicated in connection with the precept of the love of God.
Reply to Objection 1. The commandment that prescribes an act of virtue does not prescribe the mode which that virtue derives from another and higher virtue, but it does prescribe the mode which belongs to its own proper virtue, and this mode is signified in the words "with thy whole heart."
Reply to Objection 2. To love God with one's whole heart has a twofold signification. First, actually, so that a man's whole heart be always actually directed to God: this is the perfection of heaven. Secondly, in the sense that a man's whole heart be habitually directed to God, so that it consent to nothing contrary to the love of God, and this is the perfection of the way. Venial sin is not contrary to this latter perfection, because it does not destroy the habit of charity, since it does not tend to a contrary object, but merely hinders the use of charity.
Reply to Objection 3. That perfection of charity to which the counsels are directed, is between the two perfections mentioned in the preceding reply: and it consists in man renouncing, as much as possible, temporal things, even such as are lawful, because they occupy the mind and hinder the actual movement of the heart towards God.
Source: Summa Theologica (New Advent)