The Beginning

At the dawn of time, when First Lightning struck the Dead Tree and set the universe in motion, primordials spawned from the embers of this Primary Interaction and rapidly expanded outward. Most had no awareness of themselves or others around them, simply moving and colliding and destroying and growing in a heedless dance of chaos, bound tightly by the rules of the world into which they'd been spawned. Some primordials became aware, or were born aware, however. Divine goals were fashioned, Time itself arranged, and the grand game board set.
Slowly by the understanding of mortals, life pieced itself together and struggled to find a foothold in the dark. Aware of the primordials (many of whom paid them no mind), the mortals studied them intently, carefully, and painfully, veering too close at times and threatening life on entire worlds when this contact became careless. These seemingly insignificant beings marshalled the leavings of primordials and discovered fragments of their secrets, and eventually challenged them directly. These mortals became gods, mighty in the stolen ways of the primordials, but too weak to maintain this power forever. As gods fought and struggled with their newfound abilities, mortal life trembled in their wake, and most gods put aside their struggles with each other and the primordials to shepherd the mortal stock whence they'd come.
Now, the gods serve to defend mortals from an indifferent universe capable of snuffing them out. Even the elven sages have little understanding of how this is possible, though the gods themselves gain this understanding when they ascend. Apotheosis is not only somewhat common (hardly a century goes by without one being becoming a god), but necessary (this same century typically sees a god go mad). Some religions incorporate this replacement into their belief systems while others pop up and last as long as their deity. There's no easy way to predict how the burden of divinity will affect the mind, and some gods burn out after a few hundred years, while others have persisted since the start of recorded history.