6 Let us hear John: “Jesus baptized.” We said that Jesus baptized. How Jesus? How the Lord? How the Son of God? How the Word? Well, but the Word was made flesh. “And John also was baptizing in Ænon, near to Salim.” A certain lake, “Ænon.” How do we know it was a lake? “Because there was much water there, and they came and were baptized. For John was not yet cast into prison.” If you remember (see, I say it again), I told you why John baptized: because the Lord must needs be baptized.
And why must the Lord be baptized? Because many there would be to despise baptism, that they might appear to be endowed with greater grace than they saw other believers endowed with. For example, a catechumen, now living continently, might despise a married person, and say of himself that he was better than the other believer. That catechumen might possibly say in his heart, “What need have I to receive baptism, to have just what that other man has, than whom I am already better?”
Therefore, lest that neck of pride should hurl to destruction certain men much elated with the merits of their own righteousness, the Lord was willing to be baptized by a servant, as if addressing His chief sons: “Why do you extol yourselves? Why lift yourselves up because you have, one prudence, another learning, another chastity, another the courage of patience? Can you possibly have as much as I who gave you these? And yet I was baptized by a servant, you disdain to be baptized by the Lord.” This is the sense of “to fulfill all righteousness.”
Source: Tractates on the Gospel of John (New Advent)