Why Do Sneezes Come In Twos and Threes?

Sneezes never seem to be lonely. As soon as you expel your first mighty 'achoo,' there's usually another sneeze lurking right behind to follow it up. For some people, there may be two, three, or even 10 that come after that original sneeze, making for an awful lot of 'bless yous' from well-wishers nearby.
So why is it that our sneezes seem to adhere to the buddy system?