Near the end, when Kurosawa takes every extra mile to bring Kitahara out of the self-imposed exile and bring her to the last party of middle-schoolers, even hugging her before their encouraging words.
Seeing Kitahara at the train station at the end.
The fact that it wasn't Kurosawa's idea to bring Kitahara out of her exile but Sugawa's so she could apologize for her past actions showing how much she'd grown since middle school.
Kurosawa's talk with his teacher about the events that had occurred up until then. "Atta' Boy. That's my student!" can almost bring a man to tears.
Kurosawa slowly regaining his friends and making new relationships after the confession, especially when Kurosawa finally accepts Nagaoka as a friend and even says "Thank you" to him.
When Takigawa gets the courage to talk to him again and you see her side of the story is also rather sweet.
Doubles as a tear jerker as it puts the chapters inbetween Kurosawa's confession and Takigawa meeting him in the library in a whole new light. Knowing that the whole time Kurosawa had these awful things happening to him, Takigawa was there wishing she could forgive him but not being able to work up the courage to say anything, makes those chapters even more heartwrenching than they already were.
Also after the graduation ceremony when Kobayashi exchanges yearbooks with Kurosawa and tells him that he was never actually angry with Kurosawa over what he did since Kurosawa never did anything to him, only that he would've been angry if he had done something to Kobayashi's girlfriend before inviting Kurosawa to the class reunion party.
The last half of the final chapter beginning when Sugawa shows up at the store Kurosawa works at after ditching the singles mixer she went to with her friends.
The 10th anniversary 4-koma shows that Kurosawa and Sugawa had kept dating for 10 years after they hooked up at the end of the manga. Kurosawa even remembers the day he first ran into her, and as commemoration for the event he proposes to her.
Just to drive it home in the first 4-koma her name is listed as Sugawa Maiko. After Kurosawa proposes, the last panel reads "Henceforth, Kurosawa Maiko" implying she indeed accepts his proposal.