In episode 1, the ML & FL aren't linked romantically. They both have their own partners. The FL's partner after they have just returned from watching a movie, "Let's kiss each other until we break our front teeth..." Wow, I mean just wow. Oh God, what a great start! Couldn't stop laughing. Its only episode one and this guy is already so hilarious. Meanwhile the ML is proposing to his partner. But I do feel like there's a bit of imbalance between the two. His gf is so lovely and they shared a kiss, I wonder if this is going to overshadow the ML & FLs relationship in the future. But thankfully that didn't happen. Once he had decided to move on, the ML didn't give his ex any mixed signals, he was loyal to the FL.

The SFL started out great, but as she realized that the ML wouldn't forgive her, she quickly lost her way and tried to get rid of the FL (not murder her but still). Anyways since this is a kdrama, she was forgiven. She finally decided to move on.

In episode 3, when the leads and their respective exes were coming towards them, they played "You are just too good to be true" I just love hearing it anytime anywhere, reminds me of Heath Ledger singing it in "10 Things I Hate About You"
I guess they love the music too, I'm hearing it again at the end of episode 4 as well.

And what were the writers thinking when they wrote FL's ex. He seems like a overgrown toddler who was so hung up on the FL to the point of delusion. This when he was married to a beautiful woman, he was such a crappy husband to her. His wife and son were a gem though.