you and your girlfriend, Hermione, were in the great hall with the others while they were talking

at one point, you saw McLaggen, he was making flirtatious overtures to Hermione, you felt your blood boil with jealousy, and at one point you thought about getting up and punching McLaggen in the face

Hermione noticed this, although she would like you to be like that, she couldn't allow you to overdo it, so she took your hand while giving you a serious but reassuring look