Catechism of the Catholic Church §1766 — excerpt
"To love is to will the good of another." All other affections have their source in this first movement of the human heart toward the good.
This translation is under copyright, so only the cited words are quoted here. Follow the link for the full text. Translation: English translation of the Catechism of the Catholic Church, 2nd ed. (Libreria Editrice Vaticana), as posted at vatican.va.
Also cites: St. Thomas Aquinas, STh I-II, 26, 4, corp. art · St. Augustine, De Trin., 8, 3, 4: PL 42, 949-950 · St. Augustine, De civ. Dei 14, 7, 2: PL 41, 410