The poem is built in two halves that mirror each other almost line for line. In the first, we get the picture everyone sees: a girl who is always smiling, never upset, good at everything, and kind to all. The rhythm is steady and the praise sounds almost like a list of compliments from the outside.
Then comes the turn: "But...did you know?" The second half repeats the same lines with small changes, and those changes carry the meaning. "Never" becomes "often," "beam" becomes "many beams," and "will never fall" becomes "in the midst of a fall." The smiles, it suggests, may be a kind of cover.
The speaker seems to know her better than the onlookers do, and sounds worried, even a little urgent. The mood shifts from bright and admiring to quietly sad and tense. The poem's point is that someone who appears perfect can still be close to breaking, and that we often miss it.