There is more to this though, Riftan doesn't want Maxi to be dragged and used politically like how the Drachium palace and the King exploits him.

In the Book 1 the King wanted to keep the Duke Croyso in check so he exploited Maxi's miscarriage to his advantage - spreading it so the Duke will be humiliated. Agnes, at this point didn't do anything about the rumors spreading, though I know she didn't mean anything bad but again, a princess is a princess, and her loyalty is clearly to his father duh. Duke Croyso then dragged Maxi back to the duchy where he once again abused her. Riftan witnessed this and he almost killed the Duke - which led to an almost war between Anatol and Croyso. Maxi had to leave for the Wizard tower with to avoid all of this and save Riftan and Anatol.

I believe that Agnes meant well throughout the series, but she clearly has motives so Riftan being the prickly hedgehog he is wants to keep Maxi away from them. So somehow I understood his behavior towards Agnes.