The issue boils down to sodium levels. One of sodium’s jobs is to balance the fluids in and around your cells. Drinking too much water causes an imbalance, and the liquid moves from your blood to inside your cells, making them swell. Swelling inside the brain is serious and requires immediate treatment.
Sometimes babies can have issues. Their bodies are so tiny that they can’t handle lots of water. That’s why doctors say infants should drink only milk or formula. Then there are the cases like hazing rituals and publicized contests where people drink large amounts on purpose.