He found out baby tooth was his sister reincarnated, but she has no memory of him whatsoever, and now he knows he has a member of his family being nurtured by one Toothiana who ALSO hid this fact from him, he has his family, the sister who he gave everything for, the reason he's a guardian, not even know she is that reason, just thinking that Jack is a good Guardian, that his smile is warm even if his skin's a little cold, and that he's a lot of fun to be around.
Jack is feeling so many things at one, betrayal, hopelessness, yearning, anger, hate, adoration, fear, and he doesn't know what to do with himself, so he goes to Berk. To Hiccup. And Hiccup finds him, and all he needs to know is that he's here.
That Hiccup is there.