Catechism of the Catholic Church §2281 — excerpt
Suicide contradicts the natural inclination of the human being to preserve and perpetuate his life. It is gravely contrary to the just love of self.
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.