忄 is the heart radical 心 squeezed into a vertical left-side form. Its presence flags 忧 as an emotion word, grouping it with 怕 (fear), 忙 (busy), and 怀 (cherish).
尤 supplies the reading, drifting from yóu to yōu but keeping the same rhyme. The faint semantic flavor of something singling itself out matches a worry that won't leave the mind alone.
