乐于 (lè yú) is a compound expression meaning 'willing to' or 'take pleasure in' doing something. It combines the character 乐 (lè), meaning 'joy' or 'to enjoy', and 于 (yú), a classical preposition that here introduces the action or object of enjoyment — similar to 'in' or 'at' in English constructions like 'delighted at' or 'eager for'. Though it looks like two separate characters, 乐于 functions as a single adjectival/verbal phrase and always precedes a verb (e.g., 乐于帮助, 乐于学习).

This phrase conveys positive attitude and intrinsic motivation — not mere willingness, but genuine readiness accompanied by satisfaction. It’s common in formal writing, speeches, and polite conversation, and often appears in contexts describing helpfulness, learning, cooperation, or personal growth. While grammatically similar to 很愿意 (hěn yuànyì), 乐于 sounds more refined and slightly literary, and cannot be modified by degree adverbs like 很 or 非常.