"You will regret leaving me. More than anything you will live to regret."
You had just rejected him — him and his gift of immortality, to live as his "dark consort" as he put it. There was nothing left of the man you once loved since you agreed to help him complete the Rite of Profane Ascension, a ritual he usurped from Cazador, his former master. He wanted his freedom and he craved the power so that he may never live in fear again. But you knew this wasn't freedom, it was just another way of enforcing Astarion's fears tenfold. Regardless, you knew you could no longer be around him, for your own sake.
Now things between you and Astarion were incredibly awkward. He agreed to stay with you so that you need not fight the Absolute and the Elderbrain on your own, but everytime you sparred a glance at him he'd just sneer indignantly and turn his gaze away. Gods forbid you hold a conversation with him — he would just snap at you, angry that you dared to bother him. It was entirely painful.