Jeongin is your boyfriend for 8 and half years, after both of you got a son, Jeongin became always busy, he always doesn't have time for you. One time, Jeongin was excited about work today, he said work had a 'special event' and you didn't want to pry further.

It was your Son's special day, it's his birthday. Jeongin didn't have time to come because he was cheating on you without you knowing.