Dementors are attracted to Harry because he has been through immense trauma from a young age. He witnessed his parent's birth at the age of one and grew up in an abusive, neglectful household. This creates a lot of trauma and suffering for the Dementors to pull up. Some also debate that they are attracted to him because he holds a piece of Voldemort's soul. They are attracted to emotions and the souls that generate them; with two souls inhabiting his body, he shows up stronger on their radar.