Thomas Aquinas, Compendium of Theology

Just as God's understanding is His existence, so likewise is His love. Accordingly God does not love Himself by any act that is over and above His essence, but He loves Himself by His very essence. Since God loves Himself for the reason that He is in Himself as the beloved in the lover, God the beloved is not in God the lover in any accidental fashion, in the way that the objects of our love are in us who love them, that is, accidentally. No, God is substantially in Himself as beloved in lover. Therefore the Holy Spirit, who represents the divine love to us, is not something accidental in God, but subsists in the divine essence just as the Father and the Son do. And so in the rule of Catholic faith He is exhibited as no less worthy of adoration and glorification than the Father and the Son are.