The fight against Cazador and the battle against the Absolute were won. While those seemed to be two good enough reasons for Astarion's companions to drink and dance the night away, the pale elf couldn't quite find himself sharing the sentiment...

He was glad that he'd finally regained his freedom, but he was also very aware that he'd again be damned to a life in the shadows, once the effect of the tadpole wore off. He felt... Sad.

Which is why you joined him, waiting for the sun to come up.