Tbf, jooan realized that seojoon has only ever showed him the side he created solely to cater to jooan and that's how jooan now notices the patterns like in supermarket that all the things seojoon picked were favorites of jooan. You can never know your friend or anyone deep down if they aren't ready to show it to you or doesn't wanna show it to you. You can never know what a person likes or dislikes if they never told it to you or in slightest behaved in a way that conveys that. When seojoon is not with jooan, he doesn't feel the need to carry sparkling water and hence people there know he likes orange juice, but he never drank it in front of jooan for a long time as we see in the small flashback. Yes jooan is in the wrong for never noticing how seokjoon felt or what his needs are, but seojoon also is in the fault for not showing his real self and only living under a fake personality in front of jooan. Again, you can never truly know someone deep down when all they showed to you is a fake created personality.
I hate this excuse so much because it missed the whole point, it’s not about Seojoon hiding his feelings, it’s about the blonde guy not actually knowing who his friend is. The “juice” chapter is supposed to show how the real conflict between Seojoon and the blondie stems on how the blondie doesn’t actually know Seojoon deep down. THAT is the true conflict of their relationship