
They weren't even each other's "choice", just the better option. Not that they were actually given a choice . The adults in their lives are forcing this on them. You can almost smell the resentment that's coming, even though neither of them are actually at fault. This perfectly portrays why teenagers aren't really ready to form/be pushed into lifelong commitments or have children even if they are biologically capable

So he's 16 and she's 15. Her dad pushed her into an engagement then died. She was married off for survival even though she obviously would have still been grieving and had a baby by 16. Not that Harsen had it any easier. He was expected to uphold the standards of his title and hurry up and produce an heir. Neither of them were able to make choices for themselves, only the circumstances they were forced into. Neither was mature enough to honestly express their real feelings or take the time to try to understand the other.

Why does he even need to be engaged right now tho? And especially to a girl that he hasn't even talked to? At least let him go to a few balls and see who he clicks with.
Also the father had at the choice to offer a proposal on his own. Granted he had to rush the process because he needed to take over his family right then, but he got to choose who he wanted to rush and get married to. he did not have to choose the MC. He could have chosen any other girl from a high ranking family. But he choose the MC. The father isn't even giving his son the opportunity to pick a girl. He is just arranging everything and expecting his son to march along.
They both keep saying what they think the other wants to hear in order to keep their own secrets instead of being honest. That's how you end up with unsettled anxiety like him and resentful outbursts like her.