No one ever explained how dementors were created.

They don't spawn.

They don't breed.

They evolve.

From the souls of the lost

of the depressed

of the tortured.

The souls of those who get themselves addicted 

to Hogwarts

to Gallifrey

To Baker Street

To Camelot

To Camp Half Blood.

Those who suffer from 

Post Reichenbach Depression

and

EFDLOMG.

We are all destined to my dementors, 

feeding on the souls of the lost 

of the depressed

the tortured.