The sonnet which appears at the opening of Judith Guest's Ordinary People was written by Edna St. Vincent Millay. The lines used in the text are the last four lines of Millay's "Sonnet CLXXI."
But what a shining animal is man,
Who knows, when pain subsides, that is not that,
For worse than that must follow--yet can write
Music; can laugh; play tennis; even plan.
Given that the themes of the novel deal with identity, isolation, grief, and forgiveness, the lines from Millay's sonnet are appropriate. The lines raise the question regarding the animalistic nature of man, the intelligence of man, and the reasoning nature of man. This plays to the ideas raised in the novel regarding the mourning of Buck's death, Con's guilt, and Beth's lack of acceptance. The sonnet speaks to the ability of man to survive, in an animalistic fashion, regardless of what life throws at him or her. In essence, the poem highlights the fact that man will go on with life, even knowing that what he or she has faced is nothing compared to what can and will come.