why did the dark lord make the one ring?

[Lyta_Underhill]

Perhaps it is a result of his tutelage by Morgoth, since Morgoth poured forth his power into Arda, corrupting it, making it his 'Ring,' but he didn't consider the fact that, once his 'Ring' is made, he too is bound to it forever. Even after Morgoth is defeated and cast into the void, his corruption on Arda remains. So it could be a case of Sauron doing his darndest to control and affect the peoples of Middle Earth, just as Morgoth placed his mark forever on Arda itself. Both extended their power to the point where they could not personally extricate themselves from it. They are bound by their evil nature. Sauron manages to spread his corruption through the artifice of Rings to take advantage of the evil elements within the population, thus sowing another level of discord and ensuring the perpetuation of corruption begun long ago by Morgoth.

I suppose you could also ask why the Elves made the Three Rings, and how Annatar enlisted Celebrimbor in the making the other Rings if he(Celebrimbor) knew their nature. One passage in the Silmarillion hints at the unrest that made the inroad for Sauron with the Elves, so that he could use their talents in order to forge Rings of Power, and another tells of the need of Sauron to invest a great measure of power in his own One Ring:

It was in Eregion that the counsels of Sauron were most gladly received, for in that land the Noldor desired ever to increase the skill and subtlety of their works. Moreover they were not at peace in their hearts, since they had refused to return into the West, and they desired both to stay in Middle-earth, which indeed they loved, and yet to enjoy the bliss of those that had departed.

(i.e. they wanted to have their cake and eat it too...)

Now the Elves made many rings; but secretly Sauron made One Ring to rule all the others, and their power was bound up with it, to be subject wholly to it and to last only so long as it too should last. And much of the strength and will of Sauron passed into that One Ring; for the power of the Elven-rings was very great, and that which should govern them must be a thing of surpassing potency; and Sauron forged in the Mountain of Doom in the Land of Shadow.

This passage, from "Of the Rings of Power and the Third Age," suggests that the need of Sauron to pour a great deal of power into the One Ring arises from his need to control the lesser rings made by the Elves. The art and power the Elves, and specifically the Noldoran Elves, put into their creation was of such a nature that Sauron required a great measure of countering power to being them under control. The Noldor never did anything halfway!

Cheers!
Lyta