[updated every weekend]
Hiccup and Toothless ran away because they believed no one cared.
After five years, he's with his long-lost mom (yeah, you heard me) and living in peace with dragons, which is what he's always wanted. He didn't know the effect he had on the village.
After Hiccup left, Berk fell into a sort of depression. With the "nuisance" finally gone, they thought they would be better off. Soon they realized that even though he could be clumsy, he livened up the village a lot. Without him no one had a real reason to be very happy. Even though they had no proof, they assumed a Night Fury took his life.
Until a certain group of Vikings and an odd turn of events prove that he isn't dead.