甘 is a mouth holding something pleasant — the oldest sweetness. Add the tongue 舌 that tastes it and you get 甜: sweetness as an event happening on the tongue right now.
Chinese sweetness is emotional vocabulary: 甜蜜 'sweet-honey' describes love, 嘴甜 'sweet-mouthed' praises children who compliment their elders, and southern cuisine (Suzhou, Wuxi) is stereotyped by northerners as putting 糖 in everything — the eternal 甜咸 sweet-vs-salty war extends to tofu pudding.
Tongue 舌 meets held-in-the-mouth pleasure 甘 — SWEET.
甜咸豆腐脑之争 — the sweet-vs-savory tofu pudding war — is China's friendliest permanent internet battle, re-fought every breakfast.