The speaker of this poem is watching a friend from the sidelines and hiding a crush. On the surface, the two are part of a small, close group of friends, and that is all anyone sees. Inside, the speaker sets this one girl apart from everyone else. Her laugh is "music to her ears," and she admires her more than anyone else in the group.
What stands out is how the speaker treats the girl's flaws. She swears a lot and has little patience, yet the speaker says these things make her "even more" perfect. That sounds like real affection rather than a polished fantasy, because the speaker loves the girl as she is.
The mood is tender but a little sad. The smile in the opening lines feels warm, but the secrecy adds ache. The final stanza makes the imbalance clear: to the girl, the speaker is just one of the group, while the girl is her "whole world."