心 depicts the four chambers of a heart. As bottom radical it grounds 愈 in the inner-experience family. Healing happens in the heart-mind, and the comparative more-and-more is also tracked there as feelings shift in degree.
俞 supplies the sound (yú drifted to yù). The same phonetic appears in 偷输喻渝. Originally depicted a hollowed dugout boat - a faint sense of passage from one state to another fits both the more-and-more and the recovery senses.
