The vertical-heart radical on the left marks 愉 as an emotion — pleasure, gladness felt inside. It indexes 愉 with the emotional family: 怕 fear, 情 feeling, 怀 cherish, 快 happy, 慢 slow-of-heart. Every inner-state verb of feeling carries 忄.
俞 supplies the sound yú unchanged. It also adds a soft semantic flavour: 俞 originally meant assent and agreement, "yes" said by an elder — and pleasure is the heart's assent to a situation. Same phonetic in 偷 steal, 输 lose, 愈 heal-more, 逾 exceed.
