Sacrifice is the bedrock of adulthood.

She is well aware that her sister will not survive the Hunger Games. She must volunteer as a tribute to preserve her life as her older sister. 

In life, you must know when to hold your ground and when to yield. All partnerships center around sacrifice. This principle holds true regardless of whether they are personal friendships, familial ties, or intimate partnerships. You need to show others how important they are to you, and sacrificing on your goals and wishes demonstrates how much you respect them.