There's just something about how Astarion goes from thinking of himself as someone with only one talent, that he has nothing else to offer you and, inevitably, you'll find someone better and leave.
To someone who with his whole chest, heart and soul declares out loud without any hint of doubt that of course you like him simply for who he is, why is that so hard to believe?
There's just something about Astarion's personal quest having nothing to do with the main story line. How willingly walking into a vampire den to free him takes precious time away from what you actually came to the city to do. But you care enough to do it anyway.
And then he holds the power of 7000 souls in his hands, a power he's said he will share with you, something else he finally has to offer and would be proud to give you. But instead you remind him again he has plenty to offer without it, and in that moment he knows you truly didn't bring him here out of desire for his power but for his freedom. And maybe that's the last piece that falls into place which finally makes him believe you when you say he is enough.
So we all know that Tumblr is US-centric. But to what degree? (and can we skew the results of this poll by posting it at a time where they should be asleep?)