According to a soon-to-be published book about Hillary Clinton, the former first lady and senator was asked directly by Obama to stay on for an additional year as Secretary of State after he won reelection.
The book, titled HRC: State Secrets and the Rebirth of Hillary Clinton by Amie Parnes of The Hill and Jonathan Allen of Politico, describes a conversation between Clinton and Obama on Air Force One. “Would you reconsider, perhaps stay another year? Obama asked.”
The reason Clinton gave for declining to remain on any longer as secretary of state was because it was “time for her to leave public office.”
http://www.statecolumn.com/2013/11/obama-reportedly-asked-hillary-clinton-to-stay-on-as-secretary-of-state/