[Lachwen]

I can understand why Sauron would never have even considered that his enemies might try to destroy the Ring. Sauron was nothing if not power-hungry, and the insanely power-hungry often can't even comprehend the idea of not seeking power. So, in a way, Sauron saw Gandalf, Denethor, Saruman, Aragorn, and all those who were wrapped up in the Ring mess as rivals for his power. I'm too lazy right now to find the actual quote, but at one point Gandalf explains that Sauron thinks the new Bearer would use the Ring to throw him (Sauron) down and put themselves in his place; Sauron can't even imagine that they might try to throw him down and put no one in his place. I guess he sees everyone in Middle-earth as potential mini-Saurons.