Original: The election of a Democratic president in 2008 made Souter more inclined to retire, but he did not want to create a situation in which there would be multiple vacancies at once. Souter apparently became satisfied that no other justices planned to retire at the end of the Supreme Court's term in June 2009.
Simplified: In May 2009, Souter wrote President Barack Obama and said he planned to retire later in the same year.