A theory posted on Reddit suggests Dementors were drawn to Harry because they could sense the piece of Voldemort's soul living in him. Harry is an “accidental horcrux”, created when the killing curse casted by Voldemort backfired, fragmented his soul, and sent a piece of it to Harry's body.
Because dementors seek to suck out the souls of their prisoners, it's plausible that they are especially drawn to Harry because — as it is revealed in the last book — he actually possesses a fragment of Lord Voldemort's soul as well.
The Dementors caused pain and weakness in Harry simply by being near him. They didn't try kissing him until he got in the way of Sirius' arrest. Their mere presence has an effect on people, to different degrees. Harry in particular is affected by them because of his tragic past.
In Prisoner of Azkaban, Harry's reaction to the Dementors is a direct representation of the experience of depression. Dementors are guards for the prison, Azkaban, and come to guard Hogwarts when there is a risk of an escaped prisoner.
The Dementors feed off of despair. Voldemort's army spreads it, allowing the dementors to breed. The Dementors take care of the Death Eaters enemies, taking their souls, and demoralizing the rest of the population. It's mutually beneficial to both of them.
Albus Dumbledore was one who had long disapproved of the use of Dementors as guards, not only because of the inhumane treatment of the prisoners in their power, but because he foresaw the possible shift in loyalties of such Dark creatures.
The final book, Deathly Hallows, explains this. Snape HAD BEEN a Death Eater. He later renounced his connections with that group, but no one besides Dumbledore knew for sure whether he had truly switched sides, or was just faking it. Dumbledore, alone, knew the truth.
It's not exactly clear what would have happened if a Dementor had succeeded in kissing Harry Potter, because Harry had a fragment of Voldemort's soul in his body in addition to his own. It is possible that the Dementor would have ingested both Harry's soul and also the fragment of Voldemort's soul.
The Dementors made his inner feelings of hurt and abandonment come to life, along with bringing back his worst memories. As mentioned by Remus Lupin, Harry's true fear was fear itself, which the Dementors projected. The creatures forced him to feel the defeated mentality inside of him that he generally pushed down.
Behind the scenes. It is possible that chocolate is used to ward off the effects following meeting a Dementor as it has chemicals inside it that stimulate the release of endorphins, which are chemicals produced by the body to combat stress and pain; often leading to feeling of increased happiness.
The Dementor's Kiss has only been used as punishment in rare cases. Sirius Black was supposed to receive the Dementor's Kiss in 1994 after his escape from Azkaban, but he narrowly escaped from Hogwarts School of Witchcraft and Wizardry before the punishment could be administered.
Every time Harry is confronted with a Dementor, he hears the screams of his mother as she is murdered by Lord Voldemort. A horrific memory to hear... He hears this moment because Dementors feed upon happiness and cause depression and despair in anyone near them. They make people relive their worst memories.
We know from the canon that a Dementor's Kiss is the ultimate form of punishment, in which a Dementor clamps its jaws on a victim's mouth, leaving him or her in a vegetative state by sucking out the victim's soul.
And the only reason the Dementors choose to follow Voldemort is because he frees them from their duty to Azkaban, allowing them access to the richer pickings on the mainland.
Rowling has revealed that the Dementors were inspired by her own feelings of depression.
Ekrizdis is a minor yet pivotal antagonist in the Harry Potter universe. He was an immensely evil wizard who lived in the 15th century in medieval Britain, it is believed he was either the creator of the Dementors or that his insane experiments and torture caused the Dementors to first manifest.
Without thinking, Harry casts a Patronus in their direction, and he catches the Snitch easily, winning the game. Only afterwards does he discover that the Dementors were, in fact, several Slytherins, including Malfoy, dressed in cloaks with the intention of unseating Harry.
It's suggested that Harry's deepest fear being a dementor means that he really is afraid of fear itself. That makes sense for someone so brave and who faces so many challenges in his life head-on. Harry impulsively takes on tasks that might be too big for him.
In this game, it becomes clear that Draco Malfoy's Boggart is Lord Voldemort. Draco's fear of Voldemort likely comes from different reasons than others have. Most fear Voldemort taking over the world. Draco fears Voldemort intimidating and manipulating the Malfoy family.
J.K. Rowling told us, 'the mood-enhancing properties of chocolate are well known in both the Muggle and wizard worlds. Chocolate is the perfect antidote for anyone who has been overcome in the presence of Dementors, which suck hope and happiness out of their surroundings.
A Dementor attempting a kiss on Sirius is witnessed in the film adaptation of Harry Potter and the Prisoner of Azkaban, in which his soul is being sucked out his mouth in the form of a small, glowing, white dot, but the Dementors are fought off by Harry's Patronus before they can finish this.
Speaking of joy-thieves, she might have preferred to cloak herself in pink and fill her office with fluffy kittens, but Dolores Jane Umbridge was every bit as dark as the villainous figures she liked to surround herself with. Indeed, she was the one who sent Dementors to attack Harry in Little Whinging.
Later in the year, McGonagall duelled with Death Eater Alecto Carrow during the Battle in the Astronomy Tower and later learned of Snape's murder of Dumbledore. McGonagall was initially appointed as Headmistress, but was demoted after Voldemort took control of the school and placed Snape as Headmaster.
Despite his attraction to the Dark Arts and Voldemort's ideology of wizard supremacy, Snape's love for Muggle-born Lily Evans, Harry's mother, eventually compelled him to defect from the Death Eaters. He then became a double agent for Albus Dumbledore and the Order of the Phoenix.
“Dumbledore didn't want to lose his wand at that point and Draco disarmed him. So that meant that the wand gave Draco its allegiance, even though Draco never knew it, even though Draco never touched it. “From that moment on, that wand gave its allegiance to Draco, and it wouldn't work as well for anyone but Draco.”