Why do fruits taste sweeter when they are ripe?

Ripe fruit tastes sweeter because the fruit changes its stored starches and sharp acids into sugars and gentler flavors. This happens when the seeds are ready, so the plant makes the fruit softer, brighter, fragrant, and more tempting to animals.

A banana shows the change clearly: green and firm at first, then yellow, spotted, and sweet as enzymes rearrange its chemistry. The sweetness helps the plant, too, because animals eat the fruit and may carry the seeds to a new place.