First of all, let me say that I never really write reviews, but this time I got so baffled I had to.

The story evolved around 3 characters where one(Sunoh) got completely walked over because he wanted to get his happiness back, second(Jojo) stayed as messy as ever, not even bothering about how her actions would affect Sunoh. And third(Hyeyeong) got the girl just because he somehow became a certain option but we don't know why it was never made clear why Jojo couldn't choose Sunoh anymore when she swore that she'd never love anyone more than him. Yet she kept running away from him, not even trying to give him an explanation. She just pierced his heart with her words and left. I really wanted her to become more mature as the years passed, but she stayed as a victim and never came out of that state
And above all the "Love Alarm". The app remained in their daily lives even after everything.
If we think that this whole drama was about not trusting an app for choosing a loved one but our own hearts, then why did JJ go to such lengths to get the "spear"?! The application was not destroyed, nor ever used by main leads. They still relayed on the app and who rang whose alarm Although that couldn't be trusted. As Sunoh's mother said her husband still rings her alarm even tho he's cheating on her. Or we could think that this drama was about the pain of unfulfilled first love and moving on but they failed to deliver that message.

Furthermore, I knew SO & JJ had no chance(it kinda become more clear when teasers came out), I hoped they'd clear out their misunderstandings as grown-ups should. but oh boy.
I am upset about what they did to the characters and how they pushed Sunoh away. He had so much potential to emerge from a broken teen to an accomplished adult, to grow and learn self-love, but writers ruined his character. They destroyed him and then gave him a "happy ending" I'm sorry what?! he isn't even being himself around Yukjoo. He had to heal first and then get back with her. However, at least she treats him right, and really loves him so I hope she'll make him happier.
Jojo never returned the smile she stole from Sunoh. She made him cry and walked away after saying "I'm sorry" but never solving for what.

Despite being painful as hell, I really loved the last meeting scene of JJ & SO. That scene carried so much pain and nostalgia, their thoughts of what it could've been or what will never be. I wish she told him the truth behind everything right there, the entire dialogue they shared lacked honesty, and that meeting had to make them let go of each other. (mostly SO) but of course, the writers had to make Sunoh accept the confusion instead without any clarifications.

On the other hand, Gulmi had more character development than I expected, watching her chasing hearts was rather entertaining. even tho she was faking it she was learning to treat others better. (hopefully)
I don't know why nobody said anything about Dukgu, his appearance was rather short.

I must admit actors did a great job portraying their characters with what was given to them. The cinematography was wonderful, the scenes were shot beautifully, and the brightness was a bit off but it was still fine.

Edit: For those who think I disliked the ending because JJxHY was endgame let me tell you that it's not like that. Since I knew that would be the ending from a long time ago. My main concern was what they'd do to characters and how they'd end up developing them. what would happen to the love alarm & if they'd stopped using it?
JJ returning a smile to Sunoh didn't imply her dating him. All I meant was for her to give him any clue of what happened because at least he deserved it. The best she could do for a person that still makes her emotional was to give him proper answers, so he could move on. That would be a humane thing to do. (from my point of view obviously)
My (and the majority of comments) main complaint here is still the execution of characters and not giving them decent conclusions.
SO basically became the wasted character for the convenience of a writer to write the JJxHY arc, he was used as a plot device for the ending. he was portrayed as a pitiful person who tried to hold on to what made him happy in season one and in season 2 they still continued using him as just that.
the writers didn't even bother to give him proper closure at the end of the season, we see him still holding on to his feelings, but right before it ends he lets go. The writers were so bet on pulling that "it's all fine" card that they somehow thought that ending him up with someone he doesn't even love (for now) was a solid idea.

The only reason why I watched this till the end is Song Kang who is one of my fav actors. He keeps getting better and better, turning into a prominent actor. Can't wait to see his upcoming projects.
As for If I recommend this or not, watch it if you can deal with heartbreak. Season 2 has a whole different vibe from Season 1.