I remember reading a book in the 70s, so it could be YA. It is set on modern-day Earth, and the story is about a boy. His mother is sick (maybe cancer?) and he is socially awkward, so he doesn't have much self-esteem. He stumbles upon an old building (maybe a barn?) and somehow meets a person/creature who tells him that the King of Light (or maybe the King of Sun) and the Queen of Dark (or maybe the Queen of Moon?) have been at war with each other for a long time, each wanting to take over. He is told he has been given the choice over who will rule.
The boy thinks the Queen is beautiful, so he chooses her. Everything feels calm, and people who were mean to him are no longer mean. However, as he goes about his day, he sees that people become more and more sad, depressed, and lethargic. His mother in particular becomes sullen and morose. No one has any energy. Frantic, he runs back to the space and says that he wants to make a different choice...
So, the boy chooses Light. This time as he goes about his day, he sees his mother having more energy to work around the house, people are accomplishing things, etc. After a while, however, he sees that people are becoming too angry and too rough. There is too much energy and people start fighting.
He goes back to the place, frightened and confused. He says that both choices brought negative reactions, so both choices were wrong. If both choices were wrong, how could he choose?
The person/creature tells him that there is a right choice. The boy is confused. The person/creature tells him that he had another choice. He could choose to do nothing.
And that is what he chooses.
So, balance is restored and the war between Light and Dark continue. The moral is something like “balance is necessary and one must fight to keep that balance.”
I've always wanted to find the book again, but I cannot remember a title nor an author. If anyone knows what book this is, I will be eternally grateful.