Praying over the texts for Mass today, I was struck especially by the beginning of the preface:
Father, all-powerful and ever living God,
we do well always and everywhere to give you thanks.
You celebrated your new gift of baptism
by signs and wonders at the Jordan.
It's a beautiful and brilliant image, to think of God celebrating. Perhaps we don't often think of God as joyful or happy, but of course He is, and infinitely so. And perhaps we don't often entertain an image of God as the host of a party, but there is a lot in the Scripture to justify such an idea. Jesus was infamous for his attendance at dinners, and he described the final fulfillment of creation in terms of a wedding reception.
At the Lord's baptism the Blessed Trinity celebrates the accomplishment of God's single purpose in creation and redemption alike: the inclusion of the creature in the flourishing communion of divine Love between Lover and Beloved, the Father and the Son.
(It seems that this vocabulary of celebration in today's preface is particular to the American English Sacramentary. It does not read quite that way in the 2002 Missale Romanum or in any other translations I have.)