If you are being asked to write about John Donne's attitude toward love in the above referenced poem, that sounds like the prompt is asking you to look at a theme. Theme, tone, voice, mood, and author's purpose are all related to your task and generally do take shape throughout a piece.
I agree that if citing lines or using examples from the poem you should carefully watch how this changes over the course of the piece and pick quotes from the beginning, middle, and end. Often, a lesson learned and displayed through a work like that is presented in one light in the beginning, but changes by the end.
If after reading the entire poem you find there is no change in attitude and the best quotes or references come from the beginning, I would use those.