The phrase "why thank you" is an expression used to convey gratitude, similar to simply saying "thank you." The "why" in this context is an interjection that expresses mild surprise, pleasure, or emphasis, and is not meant as a question. It often adds a slightly formal, friendly, or playful tone to the thanks. Essentially, "why thank you" means "thank you very much" or "thanks a lot" with an added nuance of being pleasantly surprised or grateful.