Why did Harry hear his parents when influenced by the Dementors?

Question

In the Prisoner of Azkaban Harry Potter falls under the draining effects of the Dementors (or Boggarts pretending to be Dementors) more than once. Each of these times Harry hears voices: his mother's or his father's and (presumably) Voldemorte's.

For example, during his first lesson learning how to cast the Patronus charm he hears the voices twice:

‘Expecto patronum!’ Harry yelled. ‘Expecto patronum! Expecto –’ But the classroom and the Dementor were dissolving ... Harry was falling again through thick white fog, and his mother’s voice was louder than ever, echoing inside his head – ‘Not Harry! Not Harry! Please – I’ll do anything –’
‘Stand aside – stand aside, girl –’
‘Harry!’
...
White fog obscured his senses ... big, blurred shapes were moving around him ... then came a new voice, a man’s voice, shouting, panicking –
‘Lily, take Harry and go! It’s him! Go! Run! I’ll hold him off –’
The sounds of someone stumbling from a room – a door bursting open – a cackle of high-pitched laughter –
‘Harry! Harry ... wake up ...’
_{[Prizoner of Azkaban, UK Hardcover, Ch. 12 The Patronus, Pg. 177 & 178]}

What causes Harry to hear (or remember) the last few moments before his parents' deaths when the Dementors overwhelm him?

Answer 1

The Dementors are causing Harry ( or anyone really) to relive their worst moment. Even though it seems strange that Harry would remember the attack exactly it is possible.

Being blind, dementors sense and feed on the positive emotions of human beings in order to survive, forcing their victims to relive their worst memories over and over again. The very presence of a dementor can make the victim's surrounding atmosphere grow cold and dark, and as the number of dementors increase, so do the effects. Those that are kept in the company of a dementor for too long tend to become depressed, and are often driven insane.[http://harrypotter.wikia.com/wiki/Dementor]

Answer 2

Harry heard these voices because dementors cause you to relive your worst memories. The memory Harry heard was the last few moments of Lily and James's lives.

Answer 3

Dementors cause you to relive your worst memories. When Harry was attacked by the dementors he heard the last few moments of Lily and James's lives even though he was too young to comprehend what was happening.

He was not reliving his worst memory, he was reliving Voldemorte’s worst memory. Part of Voldemorte’s soul latched onto Harry, it is this part of the soul that is forcing Harry to relive that memory.

Answer 4

In Chapter 10 of Prisoner of Azkaban Professor Lupin explains to Harry what Dementors do:

“Dementors are among the foulest creatures that walk this earth. They infest the darkest, filthiest places, they glory in decay and despair, they drain peace, hope, and happiness out of the air around them. Even Muggles feel their presence, though they can’t see them. Get too near a Dementor and every good feeling, every happy memory will be sucked out of you. If it can, the Dementor will feed on you long enough to reduce you to something like itself... soulless and evil. You’ll be left with nothing but the worst experiences of your life. And the worst that happened to you, Harry, is enough to make anyone fall off their broom. You have nothing to be ashamed of."

Apparently, Voldemort killing his parents was one of the worst experiences in Harry's life.