why did the dark lord make the one ring?

[luthien-elvenprincess]

Another quote of Gandalf's that reinforces the fact that Sauron did not need to Ring to be victorious:

It is not needed that the Ring be found, for while it abides on earth and is not unmade, still the power that it holds will live, and Sauron will grow and have hope. The might of the Elves and the Elf-friends is less now than of old. Soon he will be too strong for you, even without the Great Ring; for he rules the Nine, and of the Seven he has recovered three. We must strike.

So, Sauron didn't make the ring to gain victory over all...why did he make it?

It was part of his nature to do such things. In the beginning, he was a assistant to Aule, who loved to make things by craft and was a skillful artisan. Later, Sauron (after turning evil) even gave himself names that gave reverence to Aule (such as Aulendil, meaning Lover of Aule).
So, making a ring of power to rule all others was just an act of his very characteristics. Part of his nature involved making things by craft, also part of his nature wanted the rule of all things.
He made the ring simply because he could...and also to aid him in his endeavor of world control.