1.) God is joyously, exuberantly, happy, because He delights in God the Son.2.) God is happy because He doesn't change and is totally sovereign, not half sovereign or a third sovereign.3.) God's personal happiness is actually a foundation for our Christian living because this will actually spill on over in mercy to us who believe.
The Father says, "Behold my servant whom I uphold, my chosen, in whom my soul delights" (Isa. 42:1). God is gracious and happy because He delights in how His loving nature is accurately reflected in His beloved Son.