quite understands why his returning to Earth would be a problem, and Valentine has to explain it to him, and it can't have been an easy thing to explain, because Valentine is the reason that he can't go back to Earth:

"I don't want to go to a world we stole from the buggers. I just want to go home."

"Ender, you're never going back to Earth. I saw to that before I left."

Ender is shocked into complete silence by her admission, and Valentine then continues to explain that she made it impossible for Ender to return to Earth for his own protection. Valentine knows all too well that Peter Wiggin would use Ender for his own plots in obtaining more power. Ender would be a tool and/or weapon wielded by Peter, and Valentine couldn't let that happen.

Peter wanted Ender back on Earth, under the protection of the Hegemon's Council. "The way things are right now, Ender, that would put you effectively under Peter's control, since half the council now does just what Peter wants."

[...]

"He had plans for you, Ender."

[...]

"Ender, you wouldn't be happy spending the rest of your life as Peter's pawn."

Valentine found a way to keep Ender off of Earth to protect him from being used by Peter for the rest of his life.

Ender cannot return to Earth in the earlier parts of the novel because he is still training for his mission to defeat the Buggers. At the end of the novel, after the war has been fought, Ender still cannot return to Earth for a few reasons. The main reason is because his sociopathic brother Peter has gained control of the planet through his influence. Ender knows that if he returns to Earth he will become a symbolic figure. Peter will likely use him as figurehead in his schemes. After a life of being controlled and manipulated by others Ender has no interest in becoming a pawn in his cruel brother's schemes. Additionally, Ender feels extreme guilt for what he has done to the Buggers. He wants to pursue some sort of redemption by connecting with the dead. This idea is explored more in the the novel's sequel.