The speaker spends most of the poem remembering a girl and the small things about her: the private smile she hid when reading or looking at a friend's text, the head-back laugh, and the dark blue hair that seemed to grow along with her confidence. Each stanza opens with "I think it had something to do with," as if the speaker is trying to work out why these details mattered so much. The tone is warm, observant and a little shy.
Then the last stanza changes everything. The speaker tells us we probably thought this was a love poem, and says that in a way it is, because self-love matters more. The poem suggests the girl may be the speaker's past self, or a version of who they wanted to be. Having watched and noticed, the speaker no longer feels so alone. The mood ends up quietly grateful and reassuring, like a gentle thank-you to oneself.