The answer is friction. Essentially, this means that, in any system where one body orbits another, their rotations will be slowed. This situation is stable. There are several possible reasons for this - the most obvious being that perhaps it always rotated this way.
Satellites can also lock into synchronous orbits and produce important effects.
Alan Craig, Shadforth, Durham. I recently saw that Denham, near Slough, was twinned with Shark Bay, Western Australia. Denham has been twinned with the town of Denham in the shire of Shark Bay, Western Australia, since 1994. It was the idea of a local resident, Andrew Shaw, who planned it to mark the centenary of parish councils in England, and has been extremely successful. Schoolchildren in both communities exchange letters, and visitors to their twin town find people half-way across the world ready to extend the hand of friendship.