OK, so you know how Harry runs into that Dementor on the train at the beginning of Prisoner of Azkaban?
The Dementor attacks him and as he passes out, he hears a woman screaming.
It's revealed later that Harry heard his mother screaming, as he was reliving his worst memory: the moment his mother died at Voldemort's hand.
This raises some questions, though. Would Harry really remember something that happened when he was 15 months old? Also, why does he pass out and nobody else does?
Redditor Starzajo has an answer to both questions, in the form of this theory:
Remember, Harry had a piece of Voldemort's soul in him the entire time, and until the graveyard, Voldemort couldn't touch Harry without feeling unbearable pain.
So Starjazo posits that even the memory of Lily's powerful act of love could cause Voldemort pain whenever Harry remembers it.
It's just another reason why Dumbledore was right about this:
