You’ll eventually learn he has a “condition,” which we won’t completely spoil here.
He also appreciates showmanship and will be put off by boring behavior.
His thirst for power (among other things) manifests in his attraction to deception and lack of compassion.

Hey, you want to show him you really care, right?
You’ll also kindly him by assisting the Goblins in the Goblin Camp.
Give him the items he desires and invite him to feed on enemies.

If it feels sinister or outrageous, it’s probably going to be a hit with Astarion.
In a normal playthrough, it may take you about 20 hours of gameplay to reach this point.
That’s everything you should probably know about romancing Asterion.
Former contributor Emily Gera also works on it.