Responses
It's explained in the novel that it was his jealousy and how he thought that if he wouldn't be there, then she would be taken by some other man and that thought made him angry, because he was considering not going to the ball.
You asked why he's getting angry so I explained :) but after reading some of the novel I also dropped it, it's getting so ridiculous and annoying... #－.－)
At first I thought from the title, the FL will be strong and active enough to avoid to be a crown princess but after reading the spoilers, meh, just a weak FL that can be dragged around by the ML. Thank goodness I was smart enough to read them.