Why did the Dementors keep attacking Harry?

Why do dementors attack Harry during the Quidditch match? Well, dementors feed on the happiness of other people, and leave them with sad memories after they suck out all the happiness. Since everybody was cheering at that time and dementors thought for getting food, they left to attack during Quidditch match.

Why do the Dementors kiss Harry?

The Dementor’s Kiss is generally considered to be a punishment worse than death. Victims are left in what Muggles would call a Persistent Vegetative State: empty shells that are still alive but irretrievably “gone”.