not surprised
katniss was the leader & breadwinner of the family
she was already giving of herself before the reaping
she was already the kind of person who would sacrifice hereself for a loved one like a parent risking their life to save their child from danger.
Evidence from the text: "I volunteer!" I gasp. "I volunteer as tribute!"
"I was not surprised when Katniss volunteered to take
her sister's place because it showed how much she cared about her family!"