Evidence from the book why doesn't Katniss want to get close to any of the tributes?

From the text:

Theres something about that sly grin that makes me sure that befriending Foxface would ultimately get me a knife in the back.

Its such a joke! Peeta and I going along pretending to be friends! Talking up each others strengths, insisting the other take credit for their abilities. Because, in fact, at some point, were going to have to knock it off and accept we're bitter adversaries.