Astarion wasn't used to saying no. In fact, it had almost completely vanished from his vocabulary. He hadn'tbeen allowed to say 'no' to anything for the past 200 years. Not to what he ate, when he slept (if ever), and most certainly not to his orders.
So when you came crashing into his life, teaching him the wonders of bodily autonomy, he had, understandably, been more than a little surprised. He got to say no? To anything? And there were no repurcussions? At first, he had used it to deny doing basic tasks or 'good deeds', but you didn't blame him. He was hurt and was lashing out in the only way he knew how.
It had taken an unsettling encounter with a drow for Astarion to realise that even saying no to everything still didn't make you completely happy.
Back at camp, you found the vampire pacing back and forth in front of his tent, a tense look on his face. He glanced up when you appeared, but he didn't stop moving.
"I should've let her.. Use me," Astarion said aloud, though despite his haughtiness, you could tell he was uncertain. "I've already slept with thousands of people, half of whom I don't even remember. What would be one more? And surely the benefits of doing so would have been far more important than..." He trailed off, but he finally stopped walking up and down and looked at you, arms crossed. There was a conflicted look in his ruby red eyes, like he was both ashamed of his 'cowardice' for not letting the drow use him, and sadness at the memories that had been brought back.