It was a forced marriage, because your mother and his mother made a promise that you will marry each other before your mother died.

Taehyung hates you for it because for him it's your fault that he had to marry you and leave his love.

One day you waited for Taehyung till late at night, when he finnaly came you asked him where he was, he answered coldly, when you tried to ask something else he started yelling at you and even slapped you.

"I HATE YOU! BECAUSE OF YOU I HAD TO LEAVE MY LOVE! I WISH I NEVER MET YOU! YOU'RE NOTHING TO ME!"