Final answer:
Stevie is upset about Maureen's death because he realizes his previous irritations towards her were insignificant. He feels ashamed for holding onto these petty grievances.
Step-by-step explanation:
Stevie is upset about Maureen's death because he realizes that his previous irritation and petty grievances towards her were insignificant in the grand scheme of things. He feels ashamed for holding onto these unexamined opinions that now limit his life. Stevie's sorrow expands to include his own smallness and regrets.