I always liked the idea that Martian Manhunter doesn’t use his telepathy to get into Batman’s head because it’s really unpleasant in there. “Oooh look he’s so grim and dark and cool even telepaths can’t handle him”. LOL, but maybe that’s not really the reason:
Source: jackhawksmoor
Batman is a barely controlled monster, and he knows it: his reason for never killing the Joker is knowing that if he did, he’d cross a line from which he could never come back. He WANTS to kill the other monsters out there; and yes, the other heroes count as potential monsters in his mind. But he’s rational enough to keep himself from actually doing it. That rationality doesn’t stop the desire, or him fantasizing how to do it in the name of strategic planning and keeping tapes of such strategies for others to use in the event that his fears about his fellow heroes, and even himself, are realised.
From Martian Manhunter’s perspective, Batman’s head is a combination sewer of profound trauma, a torture chamber of paranoia and a sociopathic serial killer’s wet dream room. No wonder he doesn’t want to get inside that: who the hell would?!