The poem is about a girl or woman who hides her sadness behind a happy face. Everyone around her assumes she's fine, but she cries alone in "dark corners, where nobody goes." The speaker seems to be an observer who sees what others miss, and who urges the reader to look more closely: "You think that she's happy, / But look again."
The image of tear tracks running from her eyes to her chin shows how long this has gone on. "Years upon years, of letting them win" suggests she has been hurt or put down repeatedly, and has never fought back. Her smile is "painted," like a mask, and she has become "a master of disguise" at hiding her feelings.
The mood is quiet, sad, and a little urgent. The simple rhymes and short lines give it the feel of a gentle warning: the truth is there in her eyes if anyone is willing to notice it.