Final answer:
Odin is upset as Balder's death signifies the onset of Ragnarök, the Norse apocalypse that leads to the rebirth of the world.
Step-by-step explanation:
Odin is upset because he knows that Balder's death is a sign of Ragnarök, the prophesied apocalypse in Norse mythology. This event is characterized by a series of natural disasters and the final battle between the gods and the forces of chaos, leading to the death of many major figures, including the gods themselves, before the world is reborn anew. The tale reflects themes of inevitability and transformation, common in many mythologies and religious narratives around the world.