Two short lines carry a lot of quiet hurt. The speaker says they were best friends, and that single past-tense verb tells us the friendship is over. We aren't told why. It might be a falling out, a slow drifting apart, or something more final, and the poem never says which. That silence is part of what makes it feel real.
The second line is where the feeling comes through most clearly. The speaker calls it "sad" to have to talk about the friendship in the past tense, and the word "have" suggests they didn't choose this. The speaker seems to be adjusting to a change they didn't want, and even the grammar feels like a small loss. The title, "Seventeen," hints that the speaker may be that age, though the poem doesn't confirm it. The mood is subdued and wistful, close to a sigh. Nothing is dramatic or loud, and the plain, simple wording makes the sadness feel honest and raw.