They'll never make me hate you, Lucanis romance.
It makes sense why it's such a long slowburn. Lucanis has been turned into an abomination. He has endured an entire year of torture in the Ossuary, and that trauma will have to be something he works through for his whole life. He's lost almost his entire family, and with Illario's betrayal, all that is left for him is his grandmother and Rook. Of course he shies away from them. He doesn't know what Spite is capable of. He doesn't know how far Spite will go--who he will kill--to get what he wants. How could he ever forgive himself if he were to hurt Rook?
And so when he finally comes to an accord with Spite, literally settling his personal demons, and allows himself to enjoy a romance with Rook, they vanish into the Fade. Gone forever, perhaps? I cannot imagine the grief. When you approach him once you escape the Fade prison, he admits that he thought he lost you forever. Killing and rage was all he had left. So when Rook returns, it's a balm to his rage. He's relieved.
It means genuinely so much to me that he trusts Rook enough to fall asleep in their arms. He's spent the entire game terrified of falling asleep. But now, a good 80 hours through this game, he trusts Rook, Spite, and himself enough to sleep in their arms. And the moment I saw his eyes fall shut, it made the entire wait worth it.