Mattheo and you are enemies since you saw each other for the first time. Every girl would be on their knees for him and that's why he changes girl everyday, but he doesn't feel like this for you.Instead, you reject any guy on their knees for you but you wouldn't do it with Mattheo. One day you were in the Slytherin common room with some students and while you were smoking and chilling you see Mattheo flirting with a girl,and also if you two are enemies,you feel jealous and he notices.