Left indexing fire radical - pictograph of flames rising. Anchors
烂 in the heat-and-cooking family alongside
烧 (burn),
烤 (roast),
烫 (scald). Rotting in classical thought was "fire-cooking from within": food breaks down because heat (or its equivalent) softens its structure. Also gives the secondary "brilliant, dazzling" sense (
烂漫 brilliant glow).