To clear things up, here’s an overview of what happened in third grade.

Kyungsoo’s been bullied severely since he was young. He was always seen as ‘abnormal’ and never had any friends. He had a crush on this guy named Minjun, but when people found out he was not only tormented for being gay but was also kicked out of school (keep in mind that this is Korea, where homouality isn’t accepted, especially in a young age).

Finally he gets to go to a new school. He’s nervous and doesn’t have much hope (kind of like Sehun here lol) but then meets Jongin, who’s really sweet and friendly. They strike up a friendship soon after. However, he’s still the same kid who loves to read and doesn’t really like to hang out with other people (he’s reclusive. I hope you guys can see the huge similarity between Sehun and Kyungsoo here - that’s why they connected so well in the earlier chapters). He’s basically only friends with Jongin, who starts to ditch his other friends to hang out with Kyungsoo more and more. Taemin confronts him about it and this is when we find out Jongin has a crush on Kyungsoo (yay! the start of a nine year long crush isn’t this exciting). Kyungsoo then realizes he likes Jongin too (it’s been about 6 months since he transferred at this point) and he goes to confess to Jongin.

Taemin and Kwanghee, however, starts to mock the two. The whole school population (they’re third graders and are immature as ) laughs as Kyungsoo gets more and more flustered. Jongin is convinced his friends are teasing him for his crush and denies it. When his friends insult Kyungsoo, Jongin mocks them by agreeing sarcastically, although it’s pretty obvious that he doesn’t agree with them a bit. Kyungsoo is already so shocked and insecure about his past bullying and Jongin’s indirect rejection that he takes Jongin seriously and runs away.

Jongin confronts him about it later on and Kyungsoo sort of avoids the subject by saying everything's fine. Since Kyungsoo never really had the chance to say anything before, Jongin assumes Kyungsoo doesn’t like him and Kyungsoo is taking everything Jongin said to heart. Jongin soon forgets about the whole thing because it wasn’t a very big deal to him, but to Kyungsoo it is really hurtful.

Taemin and Kwanghee starts to bully Kyungsoo more and more, even calling him Dodo, Kyungsoo’s old nickname. This brings back a lot of terrible memories for him and he’s terrified, but he won’t back down yet. Suddenly, Taemin pushes him into the cafeteria and he ends up crashing into Jongin. Jongin spills his lunch all over him. This was supposed to be a joke between Taemin and Jongin - the latter was supposed to have spilled it on Kwanghee and make it into a huge prank to amuse Kyungsoo, who Jongin was worried for. But Taemin screws up the plan and makes Kyungsoo the victim. Jongin doesn't think much of it and tries to help Kyungsoo up, but all Kyungsoo can think about is that Jongin is one of the bullies now and everyone is laughing at him. This is the end of their friendship.

To answer the question of why Jongin doesn’t remember is he clearly does recall the prank, but he doesn’t think it was a very big deal. It was just a prank gone wrong which Kyungsoo strangely overreacted too. Since he doesn’t know anything about Kyungsoo’s past bullying experiences and how much Taemin & Kwanghee used to torture him from behind, he only sees it as Kyungsoo getting pissed off at him and suddenly stop talking to him for 9 whole years. That’s why he’s so confused when Kyungsoo tells him the truth.

Kyungsoo, on the other hand, has been going through a lot of emotional and psychological trauma since a young age. He’s a victim of bullying so naturally he’s very suspicious of everyone. He finally managed to open up to Jongin, but when he ‘rejects him’ and ‘calls him a loser’ it breaks his heart. But he is still really dependent on Jongin at this point until the whole cafeteria prank scene. He saw it as Jongin spilling the food and embarrassing him on purpose. He already thought that Jongin viewed him as a loser, so it only helped to solidify his suspicions.

And then comes 9 whole years of a confused Jongin trying to earn back Kyungsoo’s heart and Kyungsoo simply trying to get away. Kyungsoo has been so hurt and is convinced that Jongin is just making fun of his feelings. When Jongin confesses in public, Kyungsoo takes it as Jongin re-enacting Kyungsoo’s confessions from third grade and can’t handle it.

Kyungsoo still loves Jongin, because despite everything Jongin was the first person he opened up to. But it doesn’t change the fact that he views Jongin as a bully. Jongin is just clueless to what exactly made Kyungsoo like this :(

One last thing. Luhan and Baekhyun were not making fun of Kyungsoo. They are not similar to Taemin and Kwanghee in any way. They were being Jongin’s dorky friends, but Kyungsoo is so traumatized that he’s seeing things the way he believes them to be.

I know many of you guys probably think Kyungsoo still has no excuse for being so mean to Jongin. But think of it this way: he was bullied severly and Jongin was the only person he really trusted, and Jongin (in Kyungsoo's eyes) has always thought about him the same way as all his other bullies. Jongin has been mocking Kyungsoo's feelings and has been intentionally torturing him. Kyungsoo's mindset is screwed up - he is so set on the idea of Jongin being evil and cruel that he can't see any truth in the boy.

Both boys are the victims of a huge misunderstanding. Because Jongin was uninformed about the extent of Kyungsoo's mental state, he was unable to be sensitive to it.

A important fact you guys need to remember is that Kyungsoo's hatred towards Jongin is not only a result of the one incident from third grade. It's the result of his ongoing distorted view of Jongin that has been plaguing his mind for nine years, and the fact that Jongin has been constantly reminding him of the past and has been mocking him. That's why now Kyungsoo is nearly insane with the idea that Kim Jongin is out to get him. He's so obsessed and convinced that that's the truth.

I hope this made sense guys :) Things will probably get more clear as the story goes on and more interactions between current Jongin and Kyungsoo shows up.