Conversation so far:
How did Kumar Sangakkara become captain?
In February 2009, the then captain of the Sri Lankan side, Mahela Jayawardene announced that he would step down from captaincy "in the best interests of the Sri Lankan team".
What made him want to step down?
He said he believed that it would give his successor around two years to build up to the 2011 Cricket World Cup.
Did he win the 2011 Cricket World Cup?
Days after guiding Sri Lanka to the finals of the World Cup, Sangakkara announced to the public he was stepping down as captain.

Question: Why did he step down?
Self-contained rewrite: Why did Kumar Sangakkara step down?