On April 8, 2014 Microsoft will end support for Windows XP, Internet Explorer 6, Office 2003, and MS Exchange Server 2003. If you still use XP, and according to Net Market Share, as of Dec. 22, 2013, 31% of people on the Internet still use Windows XP, then you should plan on migrating to Windows 7, Windows 8, Linux or the Mac before April 9, 2014. Or retiring.
As I see it, the main reason to upgrade is security. Another reason is performance. The bottom line is money.