Objection 1. It would seem that penance does not originate from fear. For penance originates in displeasure at sin. But this belongs to charity, as stated above (Article 3). Therefore penance originates from love rather than fear.
Objection 2. Further, men are induced to do penance, through the expectation of the heavenly kingdom, according to Matthew 3:2 and Matthew 4:17: "Do penance, for the kingdom of heaven is at hand." Now the kingdom of heaven is the object of hope. Therefore penance results from hope rather than from fear.
Objection 3. Further, fear is an internal act of man. But penance does not seem to arise in us through any work of man, but through the operation of God, according to Jeremiah 31:19: "After Thou didst convert me I did penance." Therefore penance does not result from fear.
On the contrary, It is written (Isaiah 26:17): "As a woman with child, when she draweth near the time of her delivery, is in pain, and crieth out in her pangs, so ere we become," by penance, to wit; and according to another [the Septuagint] version the text continues: "Through fear of Thee, O Lord, we have conceived, and been as it were in labor, and have brought forth the spirit of salvation," i.e. of salutary penance, as is clear from what precedes. Therefore penance results from fear.
I answer that, We may speak of penance in two ways: first, as to the habit, and then it is infused by God immediately without our operating as principal agents, but not without our co-operating dispositively by certain acts. Secondly, we may speak of penance, with regard to the acts whereby in penance we co-operate with God operating, the first principle [Cf. I-II, 113] of which acts is the operation of God in turning the heart, according to Lamentations 5:21: "Convert us, O Lord, to Thee, and we shall be converted"; the second, an act of faith; the third, a movement of servile fear, whereby a man is withdrawn from sin through fear of punishment; the fourth, a movement of hope, whereby a man makes a purpose of amendment, in the hope of obtaining pardon; the fifth, a movement of charity, whereby sin is displeasing to man for its own sake and no longer for the sake of the punishment; the sixth, a movement of filial fear whereby a man, of his own accord, offers to make amends to God through fear of Him.
Accordingly it is evident that the act of penance results from servile fear as from the first movement of the appetite in this direction and from filial fear as from its immediate and proper principle.
Reply to Objection 1. Sin begins to displease a man, especially a sinner, on account of the punishments which servile fear regards, before it displeases him on account of its being an offense against God, or on account of its wickedness, which pertains to charity.
Reply to Objection 2. When the kingdom of heaven is said to be at hand, we are to understand that the king is on his way, not only to reward but also to punish. Wherefore John the Baptist said (Matthew 3:7): "Ye brood of vipers, who hath showed you to flee from the wrath to come?"
Reply to Objection 3. Even the movement of fear proceeds from God's act in turning the heart; wherefore it is written (Deuteronomy 5:29): "Who shall give them to have such a mind, to fear Me?" And so the fact that penance results from fear does not hinder its resulting from the act of God in turning the heart.
Source: Summa Theologica (New Advent)