Odin sacrificed his eye to gain knowledge from Mimir in order to protect his people from the giants. Mimir told Odin the price for a sip of water from his well of knowledge would be a great sacrifice. Odin plucked out one of his eyes and gave it to Mimir in exchange for drinking from the well. After drinking the water, Odin gained the ability to see both joy and sorrow in order to help his people against the giants, showing his selflessness and duty to protect them.