Why did the Apostle John refer to himself as, “the disciple whom Jesus loved,”?

From INTRODUCTION TO THE GOSPEL ACCORDING TO SAINT JOHN Vol. I By William Barclay –The Beloved Disciple …

All our information about John comes from the first three gospels. It is the astonishing fact…

Source: Why did the Apostle John refer to himself as, “the disciple whom Jesus loved,”?