Why do fruits taste sweeter when they are ripe?

Fruits taste sweeter when ripe because the plant changes stored starches and sharp-tasting acids into sugars, after the seeds are ready. Softness, color, smell, and sweetness all help tell animals, eat this now, so the seeds can travel to a new place.

A ripening fruit is busy inside. The gas ethylene helps switch on enzymes that loosen the flesh, fade the green color, and build fruity smells. In bananas, much of the firm starch becomes simple sugar, which is why a speckled one tastes so different from a green one.