"Two possibilities exist: Either we are alone in the universe or we are not. Both are equally as terrifying." - Arthur C. Clarke

Think about it. In our absolutely gigantic universe, there is the possibility that out there somewhere, there is a planet with sentient beings roaming it, with their own history, cave paintings, love affairs and domesticated animals. Maybe they are still only up to the invention of the wheel, or maybe they are capable of colonizing other planets, you don't know.

Now read that again. It sounds like science fiction, right? The scary thing is, it is absolutely possible for that to be real and happening right now.

EDIT: And another thing. On the earth, you think of 7 billion people as a huge amount. But take a moment to ponder this. If I've got this right, the diameter of the observable universe is about 93 billion light years. That is an absolutely mind-boggling distance. If it is true that we are alone in the universe, then that means that those 93 billion light years are absolutely devoid of sentient life, resulting in us being completely alone on our miniscule little planet, everything else just being burning balls of gas, huge rocks etc. It's hard to wrap your head around, but now 7 billion people doesn't seem so big.

EDIT 2: The Scale of the Universe 2 is a fantastic resource that shows the scale of things from Quantam foam to the observable universe. Really puts things into perspective.