@sunmoonmoon Because the author says so.
While that answer is supposed to be a joke it's also pretty much true, I suppose, and it's been bothering a lot of people that are reading this who prefer the "princess." I don't particularly agree with some of the ways they've been saying he should just accept her, so far, though because it's pretty unreasonable. Anyway, he had already fallen for the other girl back at the school and they were getting along very well back then despite their differences in station. I forget what exactly she had done since it's been a while, but she's also helped him out a fair bit back then, and got herself in trouble with her family just to try helping him again not long ago. The woman he was already in love with is in trouble, naturally he wants to help. Said woman is also more or less the best friend of the "princess," so she naturally wants to help too even if it might mean getting her rival and love interest closer to each other in the process.