When Hoseok was 8, he went to an amusement park with his mother. They were playing hide and seek together, but Hoseok's mother, instead of hiding, left him alone with a Snickers bar. He was then sent to an orphanage, where he stayed for 10 years. Since the incident, he couldn't count to 10 out loud, and would faint at 8.
He realized if he faked a smile, he would feel better. When he was with the boys, he felt that he could actually smile. However, when Jimin fainted and Hoseok took him to the hospital, Jimin's mom pushed him away. He lost his contact with Jimin, and started to fake smile again.
Hoseok was diagnosed with Munchausen Syndrome. Munchausen Syndrome causes people to act like they're sick when they're not. Often it is used to keep attention on them. Hoseoks abandonment as a child may have caused this, as he didn't have affection in the orphanage.
When Hoseok was in the hospital, he thinks he sees his mother with a child. He runs down the stairs to catch up to her, but ends up falling when he passes out, and hurts his ankle. This causes him to stay in the hospital longer. (In another timeline, Jimin saves him from falling.)
Hoseok is trapped in the past, hoping his mother will come back. He passes out when he's faced with the truth. This leads him to say that he has narcolepsy, which we know isn't true, because he has Munchausens.