Honestly it's hard to say when and how they fell in love with each other because their characters are a bit complicated. Jun developed feelings for Yohan before that incident that happened at Yohan's house, and Yohan has pretty much always liked Jun ever since he first saw him. I wouldn't say Yohan immediately fell in love with Jun back then, it's a bit of a strong word to use as they didn't even know each other. However Yohan has had a big interest and intrigue in Jun and has always wanted his attention. All I can say is that it takes quite a while for them to be open about their feelings but we'll definetely see it as the story progresses.