Another interesting theory that I’ve come across recently and it’s to do with Harry as well as the Dursleys is that a probable reason for the Dursleys to behaved so badly towards Harry is because Harry had a horcrux inside him.
Albus Dumbledore was the one who made the connection about Voldemort having made a horcrux after what he heard Harry tell him about what happen in The Chamber of Secrets in Harry Potter and the Chamber of Secrets.
Then later on, in Harry Potter and the Half-Blood Prince Albus and Harry realized that Voldemort had made horcruxes.
As Harry was finally successful at retrieving the original untampered memory from Horace Slughorn in Harry Potter and the Half-Blood Prince.
Albus also guessed correctly that Harry had a piece of Voldemort’s soul inside him sometime after it was discovered that Harry could speak and understand parsel tongue.
Although before Harry showed his headmaster the destroyed Tom Riddle’s diary and explained how Fawkes came to him etc…
Harry’s headmaster just thought the reason why Harry could speak and understand parsel tongue which is the language of snakes is because:
Voldemort transferred some of his powers to Harry when Voldemort tried to kill him. As this discovery about Harry was made in Harry Potter and the Chamber of Secrets.
As Albus Dumbledore told Harry then that speaking and understanding parsel tongue was things that Voldemort could do.
Albus then said to Harry that Voldemort had transferred some of his powers to him (Harry).
Harry also wasn’t aware about this horcrux or horcruxes until Albus Dumbledore started talking to him about horcruxes when Albus was giving Harry private lessons.
As the headmaster knew in Harry Potter and the Half-Blood Prince that there were things about Voldemort and the horcruxes that he knew that Harry needed to know.
Harry’s headmaster had also advised Harry that he should only tell what he had learnt about in his private lessons with him to Ron and Hermione.
As Albus Dumbledore realized that Harry’s two best friends will be helping him as they have done on numerous occasions.
Also, Albus Dumbledore might have had his suspicions about the Dursleys behavior towards Harry. As he and The Order of The Phoenix was aware that the Dursleys treated Harry badly while Harry was in their care.
Though perhaps Dumbledore felt powerless because Harry had to go back to the Dursleys to strengthen his, Mother’s magic.
Although it was revealed that Mrs Figg who was a squib and a neighbor of the Dursleys and Harry’s and she had a way to communicate to Albus Dumbledore.
Also, maybe Albus Dumbledore was doing something about it but at times may had felt that his hands were tied. Then getting back to horcruxes this may be true. As Ron had explained in Harry Potter and the Deathly Hallows how the horcrux affected him. Dumbledore also told Harry what happen to his hand in Harry Potter and the Half-Blood Prince.
Some theories of fans of the Harry Potter series even had suggested the Dursley’s nasty behavior towards Harry could be unnatural.
Then I think maybe and maybe not. As Lord Voldemort or The Dark Lord was unaware that there was a horcrux inside Harry and that Vernon Dursley and his sister Marge seemed to judge people and Petunia liked to eavesdrop on her neighbors.
Vernon and Petunia were always worried about what their neighbours would think if they found out that Harry was a wizard.
As Vernon and Petunia Dursley didn’t want Harry to go to Hogwarts and they always tried discouraging Harry from asking them any questions.
As Harry asked them how, did he get the scar on his forehead? Vernon and Petunia also lied when they told their nephew that his parents died in a car crash.
Hagrid thought that was a scandal Vernon and Petunia also liked to blame Harry if anything unusual or unexpected happened.
Though Dudley suddenly became nicer to Harry sometime after Harry saved him when a dementor appeared. However, Harry’s Aunt Petunia seemed to be thinking of saying good-bye to Harry when the Dursleys were leaving to go to their safe house.
However, neither his, aunt nor uncle were that emotional, but Dudley asked his, Father as they were leaving why isn’t Harry coming?
I also came across a theory about Aunt Petunia and the theory claims that she had kept the blue blanket that Harry was wrapped in when he was a baby until the day she died.
So, this suggests that she may have felt some grief for Harry? As perhaps they never saw one, another again.
Petunia also wrote a letter to Albus Dumbledore when she was a kid as she wanted to go to Hogwarts too and this had led people to think what if Petunia was a witch.
Though she couldn’t go as she wasn’t able to do magic, but perhaps she got an owl to take the letter she wrote to Albus Dumbledore?
As even though she couldn’t do magic she was aware that there was magic, and a magical world.
Although she pretended that it didn’t exist.
Also, perhaps why Petunia wrote the letter is because she knew how proud her parents were of Lily and the rejection made Petunia call her younger sister a freak.
Then another reason for why Petunia was the way she was with Harry is because she might’ve blamed Harry for things, he had no control of like the deaths of her parents.
The Deputy Headmistress Minerva McGonagall was appalled with the idea that Harry was going to live with the Dursleys. As she didn’t seem to approve of what she saw of Petunia and Dudley.
She also told Albus Dumbledore that the Dursleys wouldn’t understand Harry. Though Albus answered her by saying the Dursleys are the only family he has.
Then why he said that is because Albus Dumbledore might’ve guessed that the reason why Harry didn’t die is because Harry’s Mum had died for him.
He also told Severus Snape that Voldemort could come back. Albus Dumbledore also told Harry much later in the series that Voldemort is aware of this magic and despises it.
He also said to Harry that Voldemort doesn’t understand the power of love and he also said that he (Harry) possesses this power.
Although I think Harry was confused with Albus Dumbledore’s revelation at the time.