There's a reason Lupin called the Dementors "the foulest creatures to walk this earth." The looming joy-suckers not only try to clamp their herpes-riddled lips onto your helpless mouth, but they then slurp every ounce of happiness from you, leaving you only with your worst memories [insert "sounds like my ex" joke here].
Harry's first encounter with the Dementors was particularly unpleasant:
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We know that Harry is more affected by the Dementors than other students, which is attributed to the trauma of remembering his mother's death. However, Redditor Starzajo has an interesting perspective on this:
Harry was 15 months old when it happened. I find it difficult to believe Harry could remember this event, no matter how traumatic.
However, as Dumbledore says, to speak of minds and souls, regarding Harry, is to speak of the same thing. This is why Voldemort can see into Harry's mind and vice-versa.
A fragment of Voldemort's soul remains inside Harry. I propose that the memory primarily comes from Voldemort. This was the event where he was ripped from his body and came closest to death, a prime candidate for worst memory.
The reason Harry blacked out during Dementor attacks is because this act of selfless love by Lily causes Voldemort unendurable pain whenever he experiences it. Like when he tried to possess Harry or touch him via Quirrel.
Harry blacks out because one part of his soul is in agony while the other is not. The Dementors bring up the memory of this act of love, which Voldemort cannot stand."
It kinda makes sense, especially if you think of the terrible memory being formed of a composite of Harry's own pain (remembered or developed as he aged) at losing his parents, and Voldemort's pain at being torn from his corporeal body.