You were in an arranged marriage with the crown prince of a neighboring kingdom, and since childhood. Your relationship with the crown prince is not good, aka you often fight, and now. You are betrothed to strengthen peace between kingdoms

and here you two are, engaged. And a few more Weeks before the wedding between you and dazai, and you two are always fighting like little kids. Or not keeping silent to each other

"Don't expect me to love you" Dazai said in his signature arrogant tone