Objection 1. It would seem that prayer is not proper to the rational creature. Asking and receiving apparently belong to the same subject. But receiving is becoming also to uncreated Persons, viz. the Son and Holy Ghost. Therefore it is competent to them to pray: for the Son said (John 14:16): "I will ask My [Vulgate: 'the'] Father," and the Apostle says of the Holy Ghost (Romans 8:26): "The Spirit... asketh for us."
Objection 2. Angels are above rational creatures, since they are intellectual substances. Now prayer is becoming to the angels, wherefore we read in the Psalm 96:7: "Adore Him, all you His angels." Therefore prayer is not proper to the rational creature.
Objection 3. Further, the same subject is fitted to pray as is fitted to call upon God, since this consists chiefly in prayer. But dumb animals are fitted to call upon God, according to Psalm 146:9, "Who giveth to beasts their food and to the young ravens that call upon Him." Therefore prayer is not proper to the rational creatures.
On the contrary, Prayer is an act of reason, as stated above (Article 1). But the rational creature is so called from his reason. Therefore prayer is proper to the rational creature.
I answer that, As stated above (Article 1) prayer is an act of reason, and consists in beseeching a superior; just as command is an act of reason, whereby an inferior is directed to something. Accordingly prayer is properly competent to one to whom it is competent to have reason, and a superior whom he may beseech. Now nothing is above the Divine Persons; and dumb animals are devoid of reason. Therefore prayer is unbecoming both the Divine Persons and dumb animals, and it is proper to the rational creature.
Reply to Objection 1. Receiving belongs to the Divine Persons in respect of their nature, whereas prayer belongs to one who receives through grace. The Son is said to ask or pray in respect of His assumed, i.e. His human, nature and not in respect of His Godhead: and the Holy Ghost is said to ask, because He makes us ask.
Reply to Objection 2. As stated in I, 79, 8, intellect and reason are not distinct powers in us: but they differ as the perfect from the imperfect. Hence intellectual creatures which are the angels are distinct from rational creatures, and sometimes are included under them. On this sense prayer is said to be proper to the rational creature.
Reply to Objection 3. The young ravens are said to call upon God, on account of the natural desire whereby all things, each in its own way, desire to attain the Divine goodness. Thus too dumb animals are said to obey God, on account of the natural instinct whereby they are moved by God.
Source: Summa Theologica (New Advent)