Because most Christians do not understand "the economics of salvation," they do not understand that not only did Jesus not need baptizing, but also that it would have been a disaster for humanity if He had needed baptizing!
If Jesus had needed baptizing, then He would no longer have been the blemishless paschal lamb sacrificed to purchase grace from loving God's perfect brutal justice, which exacts eye-for-an-eye payment for sins. God would not have "seen" his sacrifice. They would have been "as filthy rags," as Isaiah puts it.
No grace would have issued from Jesus' sacrifice, and mankind, down here on Earth, would have been a faithless, self-aggrandizing monster, because that final ingredient of humanity -- grace -- would never have been added to the mix.
So, why was Jesus baptized?
Because He was "leading the way" through the sacramental procedure.
I also think that He was telling us something about Himself and salvation theology.
How was Jesus baptized -- by pouring or immersion?
Some art shows by pouring...http://www.davidmacd.com/images/jesus_baptism.jpg
Some art shows by immersion...http://www.davidmacd.com/images/jesus_baptism.jpg
Some art shows both, to avoid the issue...http://www.pitts.emory.edu/woodcuts/1670Font/00010532.jpg
In fact, in the Gospel narratives when Jesus is actually baptized it is clearly by immersion, but when He talks about it in John he refers to it as "bathing" -- a "washing away" of sins, the logic behind pouring.
I prefer the immersion procedure myself, because of the statement it makes about Baptism theology.
Calling it "Baptism by immersion" is a little bit of a misnomer. They should call it, "Baptism by coming out of the water."
Sea water is an expression of the Abyss Type, for the Sea of Damnable Souls -- sinners.
Potable -- non-salty -- river and stream water is the River Type, for Salvation.
Salty streams, however, are expressions of the Abyss Type.
The Jordan River is, and always has been, a salty stream.
Coming out of the Jordan River dramatized that one was no longer a part of the Sea of Damnable Souls.
By coming out of the Jordan, Jesus dramatized what happens to us when we are baptized.
In His case, though, Jesus is "He Who Was Made to Be Sin Who Did Not Know Sin," 2 Corinthians 5:21. In Jesus' case, He was simply the One Made to Be Sin, from the Sea of Damnable Souls.
I'll repeat the structure I devised, then post the Bible verses which I consider to be relevant to a consideration of the named Mystery.
The Sign of the Cross.
The Apostles Creed.
One Our Father.
One Hail Mary.
One Glory Be.
One Oh My Jesus ("Oh, my Jesus, forgive us our sins," etc.)
Then, the announcement, "First Joyful Mystery: The Annunciation."
Then, one Our Father.
Then, I actually really do "contemplate the Mystery."
Then, after contemplating the Mystery, I say one Hail Mary, one Glory Be, and one Oh My Jesus.
Then I repeat the cycle, beginning with the announcement of the next Mystery, and the Our Father, followed by contemplation of that Mystery, and so on.
BIBLE VERSES TO CONTEMPLATE:
Matthew 3:13-17, but in light of John 13:5-10, and also in light of 1 Kings 18:1-46, especially Verse 44 in light of Psalm 77:16 and Exodus 15:22-25.
Genesis 7:6-16, but in light of 1 Peter 3:20-21. How? By getting wet? No. By being "rised-out-of" the "Sea of Damnable Souls" by the ark -- the Church.