When sailing towards the moonrise towers the narration mentions that the player character feels a lust for blood. I am not sure it that happens anyway or only for those who let Astarion feed.
In his dream Astarion is tempted to try drinking the blood of a humanoid, something his master forbid him to do. If the player is not Astarion that is the main reason Astarion tries to do that on him. He is testing his freedom.
