Why would he want to act cool?

he was teasing my friend at recess and the teasing was about that she likes this other guy and then my friend got annoyed and she said that she likes the guy who was teasing her(she didnt mean it). and then when the guy left, my friend started to get angry at the person who told the guy and then in the library the guy was talking to his friend and he said that lets act cool infront of her(my friend) ..........so why would he say that???