Okay, bear with me for a moment while I try to explain how this works.

Imagine that each person's life is like a drop of water. It changes as time goes by. Sometimes we are alive (as water), then we die (evaporate), and are reborn (rain) either in this age or another. Each droplet is inconsequential by itself, but when you unite thousands, millions, billions of these drops, you get rivers, ponds, lakes, oceans of water.

So, we can say in this analogy that if each drop of water is the living soul of a person, then the ocean would be the entire world full of souls. (Meanwhile, clouds would seem to mirror Tel'Aran'Rhiod, where life waits to be reborn).

Now, onto the second part of the analogy. The stick.

We know from various sources, especially Moiraine, that ta'veren are the Wheel of Time's way of correcting itself when the Pattern begins to diverge from the Age Lace's original intent. Ta'veren mingle all of the surrounding threads of life together and often produce odd happenings: impressive luck, a village full of marriages, ect. If we tie this in with the previous analogy, we can say that ta'veren are devices that can change the natural flow of a current or create whirls in the ocean to change it slightly.

If you swirl a stick in water, you get a nice whirlpool that does just that. It changes the structure of the pool slightly, and it draws water into the heart of the whirlpool where the stick lies. So, we can say that if the world is an ocean, then ta'veren are sticks that shift currents just enough to correct the ocean.

Now... the Dark One has long ago promised the Forsaken that they will be immortal and the most prominent people in the universe should he take control of the world. He yearns to break free of his prison and "recreate the Pattern in his own image" or something to that effect. Now, let's envision the Dark One as a person suspended above the ocean (odd, huh?) He is trapped outside of the Pattern and cannot directly influence events, so he uses the Forsaken to do his bidding.

However, his primary concern is the Last Battle. Why? Because it determines the ultimate fate of one thing: the alignment of the Dragon Reborn. Now, if we put the Dragon Reborn into this collage of analogies, he would be the largest stick in the ocean.

With the Dark One suspended above the ocean, would he not want to reach out for that largest stick so that he can begin to twirl the ocean to his own plans? If he controlled the Dragon Reborn, he could use the Dragon's ta'veren ability to shift the outcome of the Pattern - change it, in reality. Once he controlled the Dragon, he could begin to subjugate the other ta'veren to his will over time and control more of that ocean. Eventually, he would be the one controlling the tides with his myriad of sticks, and thusly... the ocean (Pattern) will have been remade.

Hopefully that wasn't too confusing to follow. It seems to make the Dark One's goal more rational, however. Once he controls the strongest ta'veren, that ta'veren's fate-twisting ability can be used to gain control of others until he controls the entire world.