Our bodies are made up of living organisms. We are living organisms that make up the body of the Earth--if we were a billion times larger, our solar system would be an infinitely small atom, and the Milky Way galaxy would be a single cell. If we were a billion times smaller, we might live on a planet that orbits an electron.
The point is, everything is related and proportionate in varied scales--so in that sense, we're here for the same reasons that atoms and black holes are, but we are only really aware of and value ourselves, just as those things, which we can't perceive to be antonymous organisms, aren't aware of or value us. We're too big and too small to matter, and vice versa. But none of it can exist without all of it. And we'll never know 'why' because that's just not an answerable question...what we should be investigating is how.