I keep pondering on what young meant when he said there's a difference between them in cha...

I keep pondering on what young meant when he said there's a difference between them in chapter 18, but the way I see it is that the difference between both is essentially their biggest similarity: being human

If you look closely, both of them have their optimistic and pessimistic views. Despite Jiho's depression and loneliness, and how it affects him psychologically and emotionally, he's able to see meaning and beauty in everything. Everything in life, for him, has a meaning; an higher motive of existence. Things don't just "be", they're representations for bigger pictures or deeper meanings, and because of this mentality and his introspection, the burden of his loneliness and sadness burrows deeper into him and he can't help but feel it all.

On the other hand, Young simplifies everything in life. "It is what it is" mentality. Everything exists because everything exists, there isn't always a beautiful or a fundamental deeper meaning to everything that's going on. This was highlighted when he was talking about the orchestra. To Jiho, there was beauty in the arrangement, the ambience, and the setting. To Young, he cannot think or feel what this all means, he sees things from the other darker side or more nihilistic, which is there really isn't a higher meaning to those who're performing, to them, it's just another day at work. They don't care about all of this each time. And I think this is a beautiful way to reflect on both their characters.

It's so good and rare to see manhwas or even some media portray depression in someone so full of life and art and beauty and positive outlook, while the one who is in peace with life be so nihilistic, numb, pessimistic, and emotionally constipated at times.
Because Jiho feels too much, he aches too much. Because Young put a lid on his emotions, he turned a back to his aches, which manifested as his nihilism.

I feel like i lost my point but what I mean to say is, Young thinks they're different just because they're looking at things from different angles, but in the end they're both just looking for ways to cope with their lives. While one distracts himself with art and beauty, trying to give reason to everything that's happened to him, the other tries to convince himself things just happen, and that he couldn't have done anything about the things he faced.