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Spoiler!
If you wanna know why Sungyeon’s like that towards Hosik was because the so called grandfather has a lover from the old times (i just realized it after i read this piece many times) maybe the times Sungyeon and the grandfather spent the time talking to this lover. (it will show in the end of the story so keep on reading) maybe Sungyeon see that lover to Hosik and he started to keep holding onto him, because only if you know, how sad what happened to the past "them" 
