Responses
I think haesoo meant to why he felt sad is bcause the question taku asked that day ("think abt the person you love & what if never see that person again") haesoo was maybe actually thinking of jowoon that time. And haesoo said "that day was just vague thought. Today it felt much more real. It felt like something in my life is cut out for good" meaning he thought that it bcame reality where the person he love is gone in his life(not literally gone or never see but somehow not in his life anymore)

Huh.. What did Haesoo meant? I’m dumb. I only have a cell. I’m sorry for being dumb ( ´_ゝ`)