Why can't Ender trust anyone around him?

You can't trust anybody

Ender is brought up not to trust anyone, and he often feels betrayed. For instance, Ender trusts Valentine implicitly, but she becomes the tool of the International Fleet in order to get him to continue with his training. Also, Ender realizes that he cannot always trust his friends, such as Petra--when he becomes commander and Dragon Army destroys her army, she is furious with him for months, being not much better than the other jealous commanders. Ender is not given many opportunities for friendship or trust so that he becomes fiercely independent and thus, it seems, a better commander.