USA Today:

A payroll tax increase of 2 percentage points has hit workers who have received their first paychecks of the year, and has many determining how they will cut back in 2013.

There are two reasons for this increase: 1) Keynesian economic theory says tax cuts should be timely, temporary, and targeted. These tax cuts lasted two years so that makes them temporary; 2) politicians want the tax revenue to put into the virtual Social Security Trust Fund.