Why are dementors obsessed with Harry?

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.

What is the purpose of a Dementor?

Dementors are dark creatures that consume human happiness, creating an ambiance of coldness, darkness, misery and despair. Because of their power to drain happiness and hope from humans, they have been set the duty of being guards at Azkaban, where they prevent the prisoners from having the will or ability to escape.