The speaker is a student with a crush, or a deep attachment, to someone they wish could be more than a friend. At first the mood is warm and hopeful. They laugh together, and the speaker even dreams of being her "soulmate." When the teacher sets a task to write about a special person, the choice feels obvious, and the speaker writes happily about her, "all about her."
Then the poem turns. She volunteers to read hers aloud, and the speaker realizes they are not in it. The short, broken lines, like "I realize." sitting alone, make that moment land quietly but hard.
The feeling is the sting of unequal affection: the speaker gave her a whole story, and she gave them none. Nothing dramatic happens, and the poem never says "I'm hurt." That restraint is what makes the sadness feel real. The simple, slightly imperfect English also gives it a sincere, unpolished honesty, as if the speaker is just stating what happened.