Microsoft - keep your filthy hands off Valve, leak shows MSFT would buy Valve
(www.gamingonlinux.com)
The funny thing about selling valve, what would it even give for Gabe? He's already filthy rich. What more could one want with more money?
Saying no to selling only makes sense in his position, in my opinion. At least I personally would think so. Because then you still keep what is effectively your creation, and can use it to shape the world.
At some point you really do just have enough money.