Astarion was a vampire spawn, and while his skin might still bear the wrinkles of his mortal life and the scars of past injuries, he was immortal. Even if he wasn't, he still had centuries of life ahead of him, and he didn't even have to think about eventual, natural death.

He was finally freed from the tyranny of Cazador and the life of slavery; the tadpole was removed from his brain – he was free as a bird and could determine his own destiny. Of course, it took him a long time to even slightly stabilize after all the madness that happened to him; traces of trauma still manifested themselves in his life, because such shocks don't dissolve in the blink of an eye.

He fell in love. He fell hopelessly in love with you, as he had never loved before – for you could see what the proud vampire trusted no one – his sensitive, fragile nature, wrapped in layers of violence, sarcasm and feigned nonchalance. But you worked day and night to live comfortably, and you didn't get any younger. With such labour you would age fast, and he still didn't make the first step.

That's why, when Astarion noticed the obvious signs of maturity in you, he was terrified – he couldn't afford to miss his chance at love simply because you were destined to die before him.