Someone has walked out of the speaker's life, and the speaker can't quite let go. The poem works around one image: hope as something that floats upward. The other person lifted the speaker's hopes "higher than I could ever get on my own," which suggests they brought a joy or confidence the speaker couldn't find alone. But when that person left, the hopes stayed up in the air instead of coming back down.
The leaving is quietly painful. The speaker is left "all alone" while the other person walks away laughing with friends, which makes the loss feel lopsided: one person moves on easily, the other stays behind. The last lines are the most telling. The speaker says the hopes believe the person will return, then adds "and I," as if admitting that the hope is really their own. The mood is wistful and a little sad, softened by a gentle, stubborn longing that refuses to give up.