Insomnia is the starting point here: the speaker lies in bed waiting for sleep, but can't stop thinking about one person. The poem is about unrequited love, and it doesn't dress it up. The speaker admits early on that he doesn't like her back, and the rest of the poem sits with that truth.
The mood is quiet, aching, and a little self-punishing. Short, plain lines make it feel like thoughts circling in the dark. There's a touch of humor in "Even his hair is waving at her," which shows how closely the speaker watches him and how much she notices. But the humor fades into insecurity as she compares herself to the other girl, calling her "so much better" and "so much prettier." By the end, the speaker feels she isn't enough, yet she still believes her love is the kind he deserves. That mix of self-doubt and stubborn devotion is what gives the poem its sadness.