Why can a sigma bond rotate?
Sigma  bonds  are  defined as  having their  electron  density  along the  bond  axis, while  pi  bonds  have  their  electron  density  above and  below  the  bond  axis. What this mean is that pi bonds cannot rotate the same way as sigma bonds since rotation would break the pi bond interaction. See the picture below for clarification. 



If you're wondering why it rotates then the more  freely  that  a  bond  rotates, the  more  favorable the entropy.  In  general, the  more  kinds  of  motions  and  the  more  unconstrained  those motions are, the  more  favorable the  entropy.