These are my personal theories based on evidence I have seen through both my playthrough of Dark Souls and Dark Souls 2. Some of this will seem like little more than a history lesson, but it is important to establish in order to understand where my theories are coming from.
Let's start from the very beginning. It was the Age of Ancients. The world was unformed, gray, and only the Everlasting Dragons and their Archtrees existed. Then came the Disparity. The First Flame was created, and from it came Life, Death, Light, and Dark. The Witch of Izalith claimed the Lord Soul of Life, Nito, the First of the Dead, claimed the Lord Soul of Death, Gwyn, Lord of Sunlight, claimed the Lord Soul of Light, and the Furtive Pygmy (whom I believe was also Manus) claimed the Lord Soul of Dark (otherwise known as the Dark Soul). The first three waged war with the Everlasting Dragons and won, allowing them to create a new world. Thus heralded in the Age of Fire. Meanwhile, the Furtive Pygmy had spent his time splitting the Dark Soul and spreading it. These fragments were Humanity, and would ultimately be the creation of Humans. I believe that while Lord Soul of Light was finite and could only be split a few times while retaining its own identity, the Dark Soul was infinite and could be split endlessly, allowing each piece to become new life altogether. Over time, the First Flame began to fade, and Gwyn sought to extend its life and stave off the Age of Dark. So great was his fear of the Dark, that he sacrificed himself to Link the Flame. This bought some time, but ultimately was still only delaying the inevitable. So, after an undeterminable amount of time, a Chosen Undead would need to Link the Flame as well. So began the cycle of Chosen Undead sacrificing themselves to continue lighting the First Flame. But why?
Here is my first theory. The Age of Dark is the Age of Death. Speaking to Grave Warden Agdayne in Dark Souls 2, it becomes obvious that the Dark is what all humans were once a part of, and what all life is destined to return to after death. I believe that to live is to exist in the Light, while being dead is to exist in the Dark. If this is the case, then it would explain why Gwyn was so afraid of the Age of Dark. According to many Pyromancy item descriptions, Fire is Life. So without Fire, there couldn't be life. Should the First Flame go out, all of the world would begin to die out. But while humans were once one with the Dark, and while they all have Dark within them, the Gods do not. That is why Gwyn fears the Dark. For there is no place for the Gods in the Age of Dark.
Here is my second theory. The Curse of Undead was created by Nito. It makes sense if you think about it. Just as the Witch of Izalith, arguably the most knowledgable individual on Fire, knew enough to try and recreate the First Flame, Nito, the First of the Dead, probably knew enough to create the Curse of the Undead. His lair is littered with walking skeltons and necromancers, so if the Curse was infact created by any individual being, Nito was the most likely candidate. But why would Nito create the Curse? Probably because Gwyn asked him to. To understand the reasoning behind this, we have to explore other events of the time. The Witch had failed to replicate the Flame, so a solution was still needed. We know that burning humanity and souls at a bonfire makes it stronger. If the First Flame was linked to all bonfires, would that not also make the First Flame stronger? Perhaps even make it burn longer? I believe Nito was the first to discover this, as he possessed the Rite of Kindling (likely the results of his research). So by burning humanity in a bonfire, and linking the First Flame to all bonfires, you could make the First Flame burn longer by burning humanity in any bonfire. But there wasn't any purpose in doing so. Humanity and souls were taken from after death, so to kindle the bonfires enough to keep the First Flame burning, you'd have to kill countless living beings. While perhaps a necessary sacrifice, most might not have chosen to do so willingly. Thus, Nito created the Curse that would force the afflicted, the Undead, to burn countless souls and humanity in order to keep their sanity. Meanwhile, Gwyn sacrificed himself to link the Flame, and used his own soul to kindle the First Flame to keep it burning longer. But while sacrificing humanity would be a temporary buffer, ultimately the First Flame would require a constant source of powerful souls to kindle it directly. So the prophecy was created to lure Undead to Lordran to collect powerful souls, and then sacrifice themselves to keep the First Flame burning a little longer. This is also why the Darkwraiths work to return humanity to the Abyss. The Darkwraiths seek to end the Age of Fire, but depleting the supply of humanity to burn. So they collect humanity and return it to the Abyss in order to rebuild the Dark Soul and to undermine efforts to kindle the First Flame.
So this is what I believe the Link the Flame vs Darklord ending truly boil down to. A world of endless sacrifice and tragedy, or a world filled with the peace that only death can bring.