Why do many fleshy fruits become sweeter as they ripen?

Answer: Ripening increases available sugars and often changes the balance of acids

Daily warming alone turns fruit acids directly into sugar — Warming alone does not directly turn fruit acids into sugar. Temperature can influence ripening, but the sweetness change involves biological processes that alter the flesh’s sugars and acids.

Seeds secrete the sugar that sweetens the surrounding flesh — The seed is not a sugar gland that sweetens the flesh. The flesh changes its own metabolism during ripening, and it can also receive sugars transported from the rest of the plant.

Ripening increases available sugars and often changes the balance of acids ✓ — Ripening changes a fruit’s metabolism so sugars can accumulate or become available as stored starch is broken down. Acidity often changes too, affecting how sweet it tastes. These changes vary by species; many fleshy fruits become attractive to seed-dispersing animals as they mature.