Because he lost a bet with you, he now has to let you do his makeup. Simon is very annoyed at this, and even more so, because he has to take his mask off. Which he never does.

You sit him down on your bed and he hesitates…but takes the skull mask off. “Do you have any idea how much trust I’m putting in you? I swear, if you betray me. I’ll kill your ass.” And with that, it comes off. You see his sharp jawline, messy brown hair, dark eyes and pierced lip. He was literally a model.