Because there are no coincidences in the Wizarding World.
This brilliant theory, regarding Professor Trelawney's prophecy:
Trelawney's theory states that "either must die at the hand of the other, for neither can live while the other survives". Harry takes that to mean that, in the end, either he would kill Voldemort, or Voldemort would kill Harry – but this Imgur user sees it differently. Their theory states that the only way either could die would be at the hand of the other – in other words, after killing Voldemort, Harry would become immortal. Meaning that he could never see his parents, Sirius, or Dumbledore again. Sorry for the feelings.
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And this theory, which proves that Trelawney isn't ~quite~ as useless as you might think.
In Prisoner of Azkaban, Professor Trelawney attempts to guess Harry's birth month, and she guesses that he was born in midwinter. Harry was born in July, but this Quora user points out that Voldemort – a part of whose soul was inside Harry at the time – was born on December 31. MIDWINTER.
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This theory, which explains why the Dursleys behaved so awfully towards Harry.
While the Dursley's treatment of Harry was obviously inexcusable, it kind of makes sense the way this Tumblr user explains it: Because Voldemort accidentally made Harry into a Horcrux – and, as we know from Deathly Hallows – spending time in the proximity of a Horcrux makes people mean-spirited and miserable – the Dursleys could have acted the way they did because they spend ten years in the proximity of a Horcrux.
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This theory that the Centaurs in the Forbidden Forest knew about Harry's fate all along.
This Quora user points out the significance of Harry's meeting with Voldemort in Philosopher's Stone, but the even greater significance of his conversation with the Centaur Firenze. Firenze is reprimanded by the other Centaurs, who ask him how much he told Harry – the theory states that the Centaurs knew all along that Harry and Voldemort would end up fighting to the death in the forest (as they do in Deathly Hallows) and, as is their policy, they can't do anything to stop it.
Warner Bros. / BuzzFeed
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13 Mindblowing Fan Theories That Will Change The Way You Think About "Harry Potter"