Childhood

Finn was a lonely boy ignored by everyone but his loyal dog Jake. When Jake dies, Finn falls into a deep depression and attempts suicide. Instead of dying, he ends up a coma, where he lives in a perpetual dream state. This one is courtesy of Creepypasta. Basically all of the stories in Coma Jake's head are ones he used to tell Jake before he died. A fantasy world where he was a hero, princesses loved him and he would remain forever young with his loyal friend Jake by his side. And there he'll remain until he dies. So is Finn's loneliness represented by The Ice King? This explains everything!

My opinion- i love this adventure time theory 9/10!