Objection 1. It would seem that a man ought to love his mother more than his father. For, as the Philosopher says (De Gener. Animal. i, 20), "the female produces the body in generation." Now man receives his soul, not from his father, but from God by creation, as stated in I, 90, 2;118. Therefore a man receives more from his mother than from his father: and consequently he ought to love her more than him.
Objection 2. Further, where greater love is given, greater love is due. Now a mother loves her child more than the father does: for the Philosopher says (Ethic. ix, 7) that "mothers have greater love for their children. For the mother labors more in child-bearing, and she knows more surely than the father who are her children."
Objection 3. Further, love should be more fond towards those who have labored for us more, according to Romans 16:6: "Salute Mary, who hath labored much among you." Now the mother labors more than the father in giving birth and education to her child; wherefore it is written (Sirach 7:29): "Forget not the groanings of thy mother." Therefore a man ought to love his mother more than his father.
On the contrary, Jerome says on Ezekiel 44:25 that "man ought to love God the Father of all, and then his own father," and mentions the mother afterwards.
I answer that, In making such comparisons as this, we must take the answer in the strict sense, so that the present question is whether the father as father, ought to be loved more than the mother as mother. The reason is that virtue and vice may make such a difference in such like matters, that friendship may be diminished or destroyed, as the Philosopher remarks (Ethic. viii, 7). Hence Ambrose [Origen, Hom. ii in Cant.] says: "Good servants should be preferred to wicked children."
Strictly speaking, however, the father should be loved more than the mother. For father and mother are loved as principles of our natural origin. Now the father is principle in a more excellent way than the mother, because he is the active principle, while the mother is a passive and material principle. Consequently, strictly speaking, the father is to be loved more.
Reply to Objection 1. In the begetting of man, the mother supplies the formless matter of the body; and the latter receives its form through the formative power that is in the semen of the father. And though this power cannot create the rational soul, yet it disposes the matter of the body to receive that form.
Reply to Objection 2. This applies to another kind of love. For the friendship between lover and lover differs specifically from the friendship between child and parent: while the friendship we are speaking of here, is that which a man owes his father and mother through being begotten of them.
The Reply to the Third Objection is evident.
Source: Summa Theologica (New Advent)