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I think if you're in love with your bestfriend who has been in love with someone for almost their whole life, you'll feel helpless with your feelings and just hide it, catering to your friend because you think this is the only way you can be with them. It's cruel to say that its seojoon fault for not showing his feelings, when the only thing his holding onto is being his friend and secreatry. He can be afraid that their relationship will be destroyed.

The thing is, he wasn't in love with the MC, he was in love with the idea of him. He held those memories dear, and put all his expectations and emotional needs in that person in his memories, believing that when they met again MC would fuffil all he imagined MC would be. Not only MC didn't fuffil his expectations and needs, he was already with someone else. This isn't love, is just a reflexion of Jooan's expectations.
On the other hand, you there are two things about love that you're not taking into account. 1. Love is multifaceted, you have platonic love, familial love, romantic love and so on.
2. Love can change, you can become best friends with someone you were once romantically involved with, and fall in love with someone you before considered close to you like family, love is not stagnant.
What Jooan and Seojoon have is what real love is, Jooan loves Seojoon, but it wasn't romantic, cause all his emotional expectations were placed in MC. While Seojoon love is romantic but he never made any romantic advances on his friend. But Seojoon means so much to Jooan that when ML says that it's better to let Seojoon go and be with someone else it pisses him off, but he doesn't know why yet. For him Seojoon was always "my good old friend that will always be by my side"... So now he's putting in the effort to figure it out, what is this he's feeling and if this can be romantic love so that hopefully Seojoon can stay.
And sorry to break it to you, but even if your friend already has someone he likes, if you love him and want to make it known you have to confess. People aren't mind readers, and the way each person perceives your actions is different. You can choose to hide it, but then you can't blame them for not knowing. Same thing with your preferences, you can't always pick fried chicken for dinner cause your friend likes it and expect him to know you prefer salad instead. If from the start you always pick chicken he'll probably think "wow! good thing we like the same kind of food", so you can't blame him for not questioning it.
For communication to happen one side has to be willing to voice it over, and the other has to be willing to not just hear it, but to listen. And that's what's happening right now with these two.

Okay thanks about the explanation about love is multifaceted and all. I understand it now from another angle. But you can't just say that communication is the key for seojoon. The use of food for to show similarities is not a great example. There is a consequence if he speak up about his love for him, their friendship may end in worst case scenario. Ofcourse I get that there will be nothing done if he didn't speak up, and I think seojoon chose that path as not everyone can be hopeful to be chosen when they know that the person they like have someone else they've like for a long time. Of course as you've explain that the blonde is not in love with the mc which I understand now, its not like seojoon knows that. I'm just saying seojoon should not be blame for not having the courage to speak his feelings. Its just a matter of respecting the blonde's feeling for the mc as well as protect their friendship. Not everyone can bet on losing that you know.