When John calls the people of Judeah to repent and be baptized in the river Jordan, Jesus of Nazareth comes along with them. Though he is the Son of God and had no need of repentance, yet he comes and is baptized, and during his baptism something extraordinary happens. As he exists the water, the Spirit of God descends upon him from above and he hears the voice of God tell him: You are my beloved Son, in you I am well pleased.
I ask: why does God the Father say that he is well pleased?
It is interesting to note the context of the baptism of Christ. John is calling the people of Israel to repent so that God will restore the nation's fortunes. The promise in Deut 30 is that, if the nation repents, God will put all the blessings of the covenant back upon the nation. More than that, in Deut 30.6 he promises that he will circumcise their hearts so that they can love and obey God entirely — which refers, I think, to the baptism of the Holy Spirit. But the condition of all this is the repentance of the people.
When Jesus shows up and participates in this baptism into repentance, he effectively takes the burden of repentance upon himself. God knew that any repentance humans might offer is imperfect and incomplete, yet his concern is that human beings be saved. God takes no pleasure in the death of the wicked, but rather in the salvation of the sinner. Therefore when Christ takes the burden of the people's sin upon himself, this pleases God greatly.
Indeed, perhaps this is exactly what it means to be a son of God. Jesus Christ, the Son of God, is known especially as Savior. To the extent that we are sons and daughters of God, we too occupy ourselves with the salvation of others. We pray for them, we tell them the good news, we plead that God have mercy on them, and in this way God is very pleased with us.