The speaker keeps repeating "Only 17," like someone trying to slow a young person down. At first the lines are gentle and reassuring: there are so many choices, so much time, so many opportunities ahead. Being seventeen is presented as a stage of openness, when nothing has to be decided yet and there's no reason to feel nervous about adulthood.
Then the tone shifts. "Only 17" becomes "You're only 17," and the speaker begins speaking directly to the girl herself, asking pointed questions. Why commit so soon? Why say yes to him when you don't know how you feel? Why agree to marry so young? The poem suggests she is being pulled toward an engagement she may not truly want, though the text doesn't say who is speaking.
The mood moves from calm and hopeful to worried and urgent. The repetition works like a plea, and the final line, with "seventeen" spelled out, feels like a last, heavy attempt to make her stop and think.