One day it hits Thranduil that Thorin will die. Even after they have made peace and gotten together, even after everything has started to go right, he is reminded that Thorin will die. He may survive a dragon's attack, a battle against orcs, and The Battle of Five Armies, but one day he will die. There is no escape from it.
And when he does, Thranduil knows that they will never see each other again.