Seju got into the cheating because of the worry that Sumin would leave her. Why would have she done that? Becasue she loves Sumin. Why on earth do you have to care about someone's behavior toward you that much if that person is nothing to you?

Quote of the year, ladies and gentlemen. If you love someone, make sure you cheat on them to really get the message across. That'll show them!

Seju fans will insist that the main motive behind all of her actions is love. Others who've actually read the dialogue will say otherwise. Let's just agree to disagree.

Generalising based in just one opinion is plain stupidity.