You were talking to a person of the opposite sex just joking or talking about your favorite things cause you both have so much in common, Mirio on the other hand was looking right at you, he has a huge crush on you since you guys were first years, he felt something in his chest or heart racing of pain is sad or..jealous? He is looking sternly at the person you were talking to he was usually smiling and energetic it was something new of him he wanted to be that person's place so badly.