It's because of the way he avoids her eyes when someone brings up the battles, or brushes her hand away when she traces a gentle finger down one of his many scars. It's because he locks himself in the bathroom when his body is wracked with pain from lingering curses and shouts for her to leave him alone as she pleads through the door to let her help. It's because he refuses to share his reoccurring nightmares that leave him screaming into the dark. That is why she thinks she doesn't matter to him, but she does. Because she is in his every waking thought, and the rare times he dreams, the abstract plotlines unfold and develop around her.
At night, he wraps his arms around her, folding her into his embrace and holding her closely, possessively, because he knows that any morning he might wake to find that she is gone. But there is nothing he can do, because when he looks into her eyes and thinks about letting her in - possibly reawakening her own nightmares and dragging her back down with him into the anguish - he could kill himself for even considering it.
They were all mangled war children, and they all healed in time. But he chose to cauterize the edges of his wounds and leave the center raw and gaping. He had not walked away with his head held high, bearing scars that exemplified his bravery like the rest of them. He limped away, cradling his shame like a newborn infant – holding it close and nursing it, so it would always be there to remind him how he had not earned the right to survive.
When Hermione finally went – looking as mournful and perplexed as she had all those years ago in the Great Hall, gazing at the fallen and wounded fighters of the Light – goddamn if she didn't cause the very epiphany she swore Draco so desperately needed.