When we get down to fundamentals, the reason people in the Northern Hemisphere have always felt the need for additional light in what we now call December is the fact that solar light has waned. With the Sun moving lower in the sky, each day becomes shorter and dimmer. What light does flow down is noticeably weaker, hitting our part of the world from a lower angle, becoming diminished and spread out over larger areas. Weather responds, and all life feels the change. Many plants lose their leaves or wither and die.
Some creatures burrow into holes or find shelter deep inside caves to wait out the colder months.
Humankind seeks solace in numbers. We huddle together around our hearths in comradeship, song and prayer, feeling each other's warmth and love more intimately and intensely than we do at other times of year. We tend to feel greater need for each other now than at times when abundant solar energy comes down from heaven. And in all our celebrations of life, of faith, and of each other, we light electrical and other fires to compensate for the solar energy we depend upon. Our religious reasons for all of this might differ, but buried within them is the need to rekindle our faith that the Sun will turn on the horizon to bring back warmth, abundance and the conditions we need for survival on this planet.