This verse teaches that whenever God acts, he acts in a way that pleases him.
God is never constrained to do a thing that he despises. He is never backed into a corner where his only recourse is to do something he hates to do.
He does whatever he pleases. And therefore, in some sense, he has pleasure in all that he does.
This should lead us to bow before God and praise his sovereign freedom, that in some sense at least he always acts in freedom, according to his own "good pleasure," following the dictates of his own delights.
God never becomes the victim of circumstance. He is never forced into a situation where he must do something in which he cannot rejoice. He is not mocked. He is not trapped or cornered or coerced.
Even at the one point in history where he did what in one sense was the hardest thing for God to do, "not spare his own Son" (Romans 8:32), God was free and doing what pleased him. Paul says that the self-sacrifice of Jesus in death was "a fragrant offering and sacrifice to God" (Ephesians 5:2). The greatest sin and the greatest death and the hardest act of God was pleasing to the Father.
And on his way to Calvary, Jesus himself had legions at his disposal. "No one takes my life from me; I lay it down of my own accord" - of his own good pleasure, for the joy that is set before him. At the one point in the history of the universe where Jesus looked trapped, he was totally in charge doing precisely what he pleased - dying to justify the ungodly like you and me.
So let us stand in awe and wonder. And let us tremble that not only our praises of God's sovereignty but also our salvation through the death of Christ for us, hang on this: "Our God is in heaven; he does whatever he pleases."