A lot people answer with society does or our evolutionary history does. It is true that society often through history have displayed different rules of Good/Evil or Good/Bad. Evolution can explain a good amount of it as well. But nonetheless it doesn't explain or even come nowhere close to...
Is it a sin to throw paper airplanes at the teacher?
Is it a sin to masturbate in a glass of water and give it to your sister?
Is it a sin to watch porn?
Is it a sin to watch porn and frame it on your brother?
Is it a sin to fart in your sister's face?
Is it a sin to pee in someone's soda?
Do I...