When Harry was ostracized from Gryffindor house, he was alone, and scared. Every single mocking jibe and snide comment stung at his core. He had nowhere to turn until he rediscovered the Mirror of Erised. When he looked into the mirror for at least an hour he whispered a small request, “I want to disappear.“
His request was granted, little by little, he felt himself disappearing into the mirror, until there was nothing left of Harry Potter.
An unexpected side effect of being inside the mirror, what is that he was able to see into other mirrors, and move to those mirrors.
Now Harry must survive in his new home of Every. Single. Mirror.