Season 1 and 2 have a totally different vibe.
But, I think there is a reason for this and it was done on purpose like that. Season 1 describes the puppy love of the main characters, her childhood trauma, and the friendship of the boys. Season 2, although it seems disconnected from season 1, it actually isn't. It has coherence. It's gloomy, dark, it's where conflicts are solved because the three of them are adults now. Because life is not always happy and joyful, at any age but probably much less at adulthood.

I kept wondering why Kim Jo Jo can't say at loud who she loves in S 2. So, I went back and rewatch S 1 for my 8th time, I think. And there it was the reason. She didn't trample two friends. Hye Young falls in love with her but he was too timid to confess his love. Sun Oh also falls in love with her at first sight, she didn't seduce him. And Sun Oh suspected that Hye Young liked her and he didn't care about his friend's feelings. To be fair he asked HY several times, but he couldn't say it. But after many times SO asked it, he should have known better.

Kim Jo Jo was a conflicted girl from the start because of her past, unable to vocalize her fears and doubts she, obviously, couldn't solve by herself. And after all, she only was a teenager. It will be too much to ask a teenager to deal by her/himself with a such heavy load.
Her family wasn't the most loving and caring environment, and still, she grows up decent and hard-working, knowing about life responsibilities way too early.

All her decisions are rooted there, in the unresolute past (at that time in S1). She lacks a really good friend, a good guide in life like a mother or anybody that could decently guide her to know how to deal with all her insecurities. So she became gloomier, darker, saddest. She like every other woman that suffer because of love said those words: "... I already decided that I wouldn't ever like anyone else again". But those words were said in a time frame of frustration. But life continues, our opinions may change and that's is not wrong. Is not lacking in conviction, is growing from experience because after all, all of us want to love and be loved.

Sun Oh wasn't left out there in the cold, by Kim Jo Jo. She managed it wrong, yes probably, but she was a teenager, so from her teen years she doesn't have to be perfect. His proclaimed love was more obsession than anything else. He didn't stay faithful to her, he gets together to Yook Jo showing that he didn't love Kim Jo Jo. Otherwise, he'd wait, and fight for her. He left go of her hand as soon he came back home after the accident. He could have escaped and go to see her, but he didn't. So, she wasn't his choice either. His love was selfish, loud, and violent. He couldn't have a decent conversation with her after they broke up, he only could express himself yelling at her. Meanwhile, Hye Young was gentle and his love although silent was steady from start to end. Of course, he also has his own conflicts, but who doesn't? His love was real, not in the spur of the moment, no just curiosity. He respects her, all the time. He acts based on what will hurt Kim Jo-Jo and not based on his own selfish motives. Because he truly loved her from the start.

Sun Oh didn't lose his smile, we can see him smiling pretty often with his new girlfriend Yook Jo.

The casting was excellently done. The chemistry was there: between the main leads, in their respective roles as girlfriend-boyfriend and as friends too.

The acting was really good, all of them. The crazy, rude, unreasonable cousin was very well portrayed.

I also liked her drawing in The Ringing World. I liked how they depicted her gloominess.