Why does sautéing onions make them sweet and soft?

When you sauté onions, heat breaks down their cell walls, releasing moisture and causing them to soften. The sharp, pungent flavor comes from sulfur compounds that are volatile and largely evaporate or transform with heat, removing much of the harshness. The sweetness develops through two processes. First, the onion's natural sugars, which were masked by those sharp sulfur compounds, become more prominent as the harsh notes disappear. Second, at higher temperatures, caramelization occurs, where sugars break down into hundreds of new flavor compounds that taste rich and sweet. The Maillard reaction also contributes, creating complex browned flavors when amino acids and sugars interact. Together these changes turn a sharp, crunchy vegetable into something tender and mild.