I thought it’s relatively obvious that the ascended ending sets up Astarion as the next Cazador and, in case of a vampire bite, a “master - slave” dynamic (same one Cazador and Astarion had) between Astarion and the player, hence the dialogue options.
The narrative also makes it clear that his story is about abuse (it’s really explicitly stated) and it would be a bit weird to be happy about him never getting out of the abusive circle.
I don’t know what OP’s RP reason was for ascending Astarion, but there are a few answers here, which made me think some people understand the ascended relationship as a consensual sub-dom dynamic, and it’s just not the story.
