Balance. Order. Equality.
Nature abhors a vacuum; too much of one thing leads to too little of another.
If a river feeds all its water to the ocean, what is left to flow? But if an ocean never meets the river, how can it get its fill?
If the sun shines every day, the land will become parched. If it rains every day, the plants will wither and die.
One who always laughs will never appreciate their joy. One who always cries will never know the depth of their sorrow.
If good always triumphs over evil, how can good have any worth? If evil corrupts all good to the core, how can it have any impact?