He knows he shouldn't be laughing right now...You're working so hard to ease his muscles and treat his knee but the more you scold him, the more he laughs just because of how adorable you look...

You've been taking care of him ever since he injured himself in the first place...Granted you are his girlfriend so it was no big deal to you, but to him, it meant everything...He could never get over how adorable you look taking care of him, which is why he's now dodging your slaps as he laughs...