It's not surprising but Yohan is a red flag. I mean, Jun isn't perfect himself but Yohan has some serious temper/ control issues and his mood can switch with just the blink of an eye, so he has nasty mood swings too. He'll be very up and down with Jun, cling to him if he gives him attention and push him away if he doesn't. From what some of you might be able to tell Yohan seems to like it when Jun compliments him or defends him, I personally think it's because deep down Yohan is actually very insecure about himself. This is something we'll learn later on in the story as we get to know Yohan as a character better and see more of his flaws. But as I said, since he seems insecure his sudden friendliness to Jun just now could be because he just stood up for Yohan by saying he's pretty smart and ranks well academically. Either way it takes long for the two to actually get together and communicate more but they will have quite a lot of tension between them until we get to that point.
Spoiler!
And for those who are wondering, no, we won't see them making love to each other anytime soon.