St. Thomas Aquinas (d. 1274) took up the question of why St. John the Apostle referred to himself as "the disciple whom Jesus loved", in his commentary on John's Gospel:

"Jesus did not love John to the exclusion of others, for he had said earlier [to all the Apostles], in John 15, 9: 'As the Father has loved me, so I have loved you.'  But he did love him with a particular love beyond that of his love for the others, and for three reasons.  First, for his penetrating understanding, for teachers love their understanding students.  Proverbs 14, 35: 'The wise minister is received by the king.'  Second, on account of the purity of his chastity, for he was a virgin.  Proverbs 22, 11: 'He who loves purity of heart, has the king as his friend because of the grace of his words.'  Third, because of the tenderness of his age, for we feel more sympathetic towards youths and those who are weak, and we show them signs of intimacy.  So also did Christ in the case of John's youth.  Hosea 11, 1: 'When Israel was a child, I loved him.'  Here we understand that God especially loves those who serve him from a young age."