It was perfect, King Ghidorah was dating their immortal friend Godzilla, and no mortal would ever have her. Kong, finally free of his guilt, arrives in the Earth Defenders to learn how to be more social from Godzilla and her friends. Kong fell in love with Godzilla, and Godzilla realizes that she's been missing out on the feeling of true love.
King Ghidorah isn't okay with this turn of events, not in the slightest.