They always tell you that you shouldn't sleep with your best friend, and if you seriously think about it, maybe they're right. The chance of it ever going anywhere is slim to nothing, no matter how close you are with someone. There's a higher chance of it going downhill rather than any higher than it already was. When you're that close with someone, not in the metaphorical sense - literal sense, you cant help yourself. Words get thrown around that may or may not be meant, the touching leads to more and more of it, kisses get shared, and once all of that happens, its all left to instinct, not feeling. The heated pants, the drawn out moans, the reassuring shushing, all of that wasn't based on feeling, but on heat-of-the-moment based instinct.
Karkat should have listened to them, but the feeling of it all got the best of him. The story on how it actually happened could be classified as "cute" or even "sweet", but if Karkat seriously stopped to think about it, he wishes it didn't happen. It was the end of summer vacation, and everyone was about to start school again. Dave threw a party. No alcohol was involved, however nearing the end of the party, John and Karkat left early. They went back to Johns house, and they got a little too close for the "just friends" range. John took his time, he was gentle and didn't rush anything. Hell, the whole of it all lasted about 2 and a half hours. John spoke to him softly, kissed him like they've done it so many times before, however they never have. Karkat got lost in it all, and didn't realize his best friend was just wanting to get into his pants - or that's what he thinks at the moment.
John, on the other hand, cant stop thinking about it. The closeness, the way Karkat clung to him if he got too heated, the way Karkat said his name - all of it sent chills through him. John has been with a few people before, but the feeling was so incredibly different with Karkat. He felt loved, and wanted, not to mention each kiss that they shared through all of it made his heart stop. The only thing that bugged him was before they did anything, Karkat had said something about how they shouldn't do it again, and that they needed to remain best friends, which he had agreed to. That was the only thing he regretted, which was agreeing to not change thing. So whatever feeling John put into that night, it would mean absolutely nothing afterwards. He was right about it too, since right after they had finished finally, Karkat went home and John was left to clean everything up by himself. Which bugged him, of course. It bugged him a lot knowing Karkat seemed to just not even care about it.
Ever since they did that, Karkat noticed John stopped talking to him as much, and he was always staring off into space, not listening to anyone. John noticed Karkat was irritable, and he was getting incredibly snappy at everyone like he did before. He didn't like getting hugs, he didn't like it when anyone tried to ask him about when they did anything. In fact, he yelled at Jade when she simply asked what was wrong with him. They would argue a lot, too. They argued all of the time, about stupid things too. John laughed to loud, Karkat got annoyed, they started arguing. Karkat got rude with someone, John called him out of it, Karkat got offended, they argued.
Despite how much they argued, and how they were seeming to drift apart, they still would hang out together after school, help each other with their homework (Karkat sucked at English, John sucked with History), just like they always did. They promised to remain best friends, and they were both gladly hanging onto that. Even if John feels like Karkat hates him and Karkat feels like John is using him. Since John had his feelings for Karkat sealed away at this point, he agreed to go out with Roxy, and he seriously thought he was into her. However, Rose (being the wonderful lady she is) could see that John was trying to mask his feelings, and act like they weren't there at all. Karkat was left to believe that his suspicious were right, since John was conveying the impression all he wanted was the sex. Which he hated.
He knew that this school year was going to be a long one, just from all the drama that happened in the first week of school.