Jisung's hates Chenle. There's nothing more to it, or is there. He says he hates him, but does he really? He should, Chenle's so annoying. But Chenle is also dying. Damn you heart why do you do this to me, he thought. I hate you, but I love you. How does that work? Perhaps the feelings you have for a certain person aren't really your true feelings. But who knows? Only you control your thoughts. Chenle on the other hand, he loves Jisung. So much it probably isn't healthy for him. He knows the younger hates him, but he doesn't care. 

A crush is unforgetable. 

Your first love is irreplaceable.