In the hunger games why does the narrator volunteer?

To save her sister

Explanation:

The reason Katniss volunteers is, of course, to save Prim, her little sister. Despite the odds being in her favor, Prim is selected by the lottery system that decides which children become tributes. Because she is just twelve years old, and because she is a sensitive, nurturing person who has difficulty with any kind of suffering or violence, she is almost certain to die in the Games.