The combination 费心 (fèi xīn) beautifully captures the idea of “spending one's mental energy” or “investing one's heart” into a matter. It implies a deeper level of care and consideration than simple physical labor.
In Chinese culture, acknowledging the efforts of others is a cornerstone of politeness and building good relationships (关系 - guānxi). Simply saying “thank you” (谢谢 - xièxie) can sometimes feel insufficient, especially when someone has gone out of their way for you. Using 费心 (fèi xīn) demonstrates a higher level of social awareness. It shows that you not only appreciate the result but also recognize and value the unseen process: the planning, thinking, and caring that went into the action. Comparison to Western Culture: The English phrase “Thanks for going to the trouble” is a close equivalent, but it can sometimes carry a slight connotation of apology, as if you've caused an inconvenience. 费心 (fèi xīn), however, is almost exclusively positive and appreciative. It focuses on the giver's generosity of spirit and thoughtfulness rather than the receiver's imposition. It's less “Sorry to be a bother” and more “I'm deeply touched by the care you've shown.”
费心 is common in both formal and informal situations, but always carries a sense of sincerity and respect.