Why is katniss upset about the person who's joining her in the games

Katniss feels she owe Peeta for the kindness that he showed her when she was a child. He was also a child and, being the baker's son, gave Katniss bread when she was hungry.

Katniss feels she owe Peeta for the kindness that he showed her when she was a child. He was also a child and, being the baker's son, gave Katniss bread when she was hungry.

I think also, he is from her district and killing him would not go over well with anyone.

After Katniss' father's death the family was hungry and had nothing to eat. Peeta was only a little boy, but he gave her two loaves of burnt bread (I should say threw them to her), and she later realizes that he probably burnt the bread on purpose so she could have it. He's always been a good memory for her and his act was a reminder that there's always hope. She's upset that he's joining her in the games because it makes him her enemy.