Why did J.K. Rowling write Harry Potter and the Deathly Hallows?

Philosopher's Stone - June 30, 1997 (UK)

Sept 01, 1998 (US)

Chamber of Secrets - July 02, 1998 (UK)

June 02, 1999 (US)

Prisoner of Azkaban - July 8, 1999 (UK)

Sept 8, 1999 (US)

Globet of Fire - July 8, 2000

Order of the Phoenix - June 21, 2003

Half Blood Prince - July 16, 2005

The Deathly Hallows - July 17, 2007