Perhaps no human question has been picked over so many times as the one concerning the destinies we're meant to fulfill while here on earth. The typical inspiring answers invoke morality, duty, and conscience. Human destiny is described as a journey to perfection of some sort. The trouble is that nobody ever comes close to achieving any sort of destiny mapped out for them by the idealistic exhorters.
I have a realistic answer to the question, "Why are we here?" The answer is that we're here to feed the plants.
Perhaps I should explain in more detail. Life on earth exists because of a complex set of energy transformations that is sometimes described as a food chain, but is more properly called an energy web. In its simplest form, it consists of two complementary chemical reactions, Reduction and Oxidation. In the broadest view, Reduction converts kinetic energy to potential energy, analogous to pushing a stone uphill. Oxidation converts that potential energy back into kinetic energy, or letting the stone roll downhill again.
Reduction can happen in a number of ways with any energy source, including geothermal vents at the bottom of the sea. For the sake of simplicity, we'll consider the most familiar example: photosynthesis. The energy source is sunlight and most of the potential energy is stored as complex carbohydrates in the plant body:
Energy (sunlight) + carbon dioxide + water + nutrients --> carbohydrates + oxygen.
So Reduction is the job performed by plants. Animals, on the other hand, oxidize the plants with the reverse reaction:
Carbohydrates + oxygen + water --> energy + carbon dioxide + plant nutrients.
We use the energy to keep us breathing and moving, but we're also sustaining the plants. Nutrients are sequestered in the plant body along with carbon. Without oxidizing animals to consume them, vegetable material piles up until all nutrients are used up (which happens sooner than carbon dioxide depletion), at which time the Reduction process, and thus plant life, stops. In contrast to some theories of origins, it's obvious that both chemical reactions have had to occur simultaneously since the beginning of life.
The oxygen / carbon dioxide cycle between plants and animals is common knowledge. The nutrient cycle is equally important, but seldom noted. Plant nutrients include water-soluble nitrogen, phosphorus, and potassium compounds. In the ocean, these nutrients move around, but on land they are stored in the earth and supplied to plants through their roots. In unbalanced ecosystems where natural plant decay processes are not allowed to happen, these nutrients must be supplied by artificial fertilizer. In a balanced ecosystem, much of this nutrient recycling takes place in situ, where microbes and insects in the earth eat the dead plants, or fire, that fast oxidizer, converts the plants to ash. But more mobile animals can spread nutrients to locations where there may not be enough occurring naturally.
This is where humanity finds its role. As animals, it's our duty to breathe out carbon dioxide and to provide plants with essential nutrients. Let me clarify one important point: despite the phrase "pushing up daisies," you don't have to die to provide nutrients. In fact, that's quite inefficient. Only about 5% of our bodies are made up of compounds that become plant nutrients, so a 150-pound corpse contributes only about 8 pounds of nutrients, and even that is applied very locally. That amount won't support very many daisies.
But consider excretion: it consists of concentrated plant nutrients. If a human produces 2 pounds of urine and feces with a 10% nutrient content per day for 70 years, a single human being produces, in a lifetime, 5100 pounds, or 2 1/2 tons, of plant nutrients. When combined with carbon dioxide exhalation, that's an impressive amount of help in feeding the plants. It's our duty.
And that's why, your honor, I was urinating in the bushes in the park last night when your constable so rudely apprehended me.