Why not? Its not like he has any financial worries. And the company was in a position where he has all the people in place to run it smoothly. Im not sure if those are his reasons but they seem plausable.
He retired so that he and Melinda Gates (his wife) could rub the Bill and Melinda Gates foundation full time and basically give away most of his billions of dollars. As Kerosene said, he is working full time on getting a Nobel Prize. Hell if Al Gore can do it so can Bill Gates.
After thirty years of trying to keep the ship afloat, maybe he just got tired of it. So now he is hanging out with Melinda at the foundation.
Bill Gates have been the richest man on earth for a long time and he decided to give the money back to the society. So, right now, he want to focus with his Bill & Melinda Gates foundation.
Well.., he was not working too much anyway, but he was the lider. But it is a job too, and a lot of stress. Now I got the answer.
here is the reason http://freehumours.blogspot.com/2008/08/bill-gates-decided-to-sell-off.html, quite funny though
There was an interview with him on the BBC a few weeks ago and he said he wanted to spend more time on charity work.