As a boy of high status, he's never left wanting, so he's never had to struggle with life. Anything he wants, he can get. Then out comes this princess of low status, who is a blemish to the family. Yet she has the pride and confidence that can put even a pure royal like her sister to shame. And would even reject him.
So he starts thinking that she's the missing piece of the puzzle. The woman that can make him want something for once in his life. Of course, he still has arrogance in him by thinking about status and not an emotional relationship because he was raised in a society where the station of one's birth dictates everything. So he thinks that naturally, she would want to be with him because she will never be able to connect with the other family members and always feel ostracized.
While I don't believe Phillip is a "bad" person, he's too arrogant and needs to be humbled. And no, I don't just mean that Amon has to be a little sh*t like he was during this chapter of being such a petty jealous person to ruin her gift, but I mean also have the case of the father also working on his relationship with his daughter and actually acknowledging her, showing that he will actually fight against the great arrogance of status to give the daughter born from the woman he loved some shred of happiness.