Life is full of cycles, opposites, complements, and balance. The Earth revolves around the Sun, the Moon revolves around the Earth, and the Earth rotates to create a day-and-night cycle that many animals have evolved to set their periods of activity and rest to. There are also complements in nature such as men and women, sunshine and rain, effort and rest, and meat and vegetables. Finally, there are opposites like hot and cold, light and darkness, and north and south magnetic poles. Life is full of extremes, variations, and the balance between them.
Why does life contain these differences? First, life is full of contrasts. There's a silly-but-profound part of the cult classic movie, Vampire in Brooklyn, where Eddie Murphy's character says, "If every day was a sunny day, then what's a sunny day? You can't have a sunny day if every day is a sunny day. There's no contrast, there's no excitement, there's no thrill. If every day was a sunny day, you wouldn't appreciate the sunshine." The punchline of the scene is that he gets a whole church choir to sing, "Evil is good!" because he convinces them without evil there would be no good.
And that's the reality of life: it is full of diversity and contrast for you to find things that you want and need in life. Some are due to evolution: cells reproduced into more complex organisms, and when competition between those organisms got fierce, genetic mutations would create new organisms that could better compete by eating other organisms or simply being more fit at acquiring the resources they needed. It is true that an organism needs both happiness and sadness to know when something is good or bad for it. Life has evolved so that it benefits from both sunlight and darkness so that many need both to function optimally (example: night-shift work has a higher mortality rate in humans). Specifically, humans have evolved to seek novelty for the potential it has to assist in their growth.
While it's important to optimize growth in your life to reach your full potential, life is often very much a balance between various elements vs. trying to isolate and maximize one thing. For example, most people find that a healthy balance of foods will provide the nutrients they need vs. a restricted diet (note: there are exceptions, of course), and even the top-performing individuals in a field have limits to what they can sacrifice in other areas without it hurting their performance in their primary interest. Diet, exercise, sleep, and social activity are all things that can help keep a person performing at their best as they try to be the best athlete, academic, or businessperson possible.
As a result, there are advantages to balance, opposites, cycles, and complements. Animals use the seasons to balance rest with activity. Sexual reproduction evolved as a way for animals to have two types of a species that could specialize in hunting and protection vs. nurturing and development. Humans use heat when the weather is cold and air conditioning when the weather is hot. Opposites and balance allow people to find what they prefer in life and use it to their advantage.
You can take advantage of this in many ways: you can buy stock during down business cycles and sell during bull markets. You can use your strengths to make up for others' weaknesses and leverage others' to make up for yours. You can use light for activity and darkness for rest. You can balance work and play, friends and family, and professionalism with silliness. It's up to you how you combine all of what is available to you in life, but know that your life doesn't have to be one extreme or the other. You don't have to commit or restrict yourself to one thing. There are plenty of things you can do, and there's nothing wrong if you wish to become a specialist or generalist. But no matter what you do, there will always be things that can help or hurt you, good or bad things you can do, and multiple competing options that you can select from or combine to make your life as best as you can.
For every Yin, there is a Yang. Balance will keep you sane, and complements will fill your needs. Embrace the diversity in life—in people, places, and things—because you never know when something that is different to who you are or different to what you've ever seen before might just be the thing that fills the hole in your life right now. When that happens, you'll understand why variety needs to exist: different people need different things at different times to have their optimal life experience.