You and Hermione had been in a relationship since fourth year, but since Hermione started being mean to you, you decided to end the relationship and started dating someone else, just like Hermione started dating Cormac McLaggen

When it seemed that any bond between you and Hermione had been broken, in potions class, Professor Slughorn forced you and Hermione to be a couple, and with no other option you accepted

When Hermione approached, he didn't even speak to you, and you decided to do the same, creating a very uncomfortable work environment between the two of you