Why does Ender think that Graff who is an adult is his friend?

Ender has perceived Graff as a friend because, he says, Graff has not lied to him. It is interesting that Ender seems to think that someone who is truthful must be his friend, so perhaps Ender is lying too--we know that he sees lies as sometimes more dependable, and Graff seems to be one of those who is willing to lie to Ender to achieve the result he wants.