-Humanity is the first proper civilisation, and therefore, probably will not see other civilisations in a very long time and if we do, they would be primitive, like cave men.
-Humans are an anomaly. No other intelligent life in the universe. Sad face.
-There were always aliens on Earth, but when they found out about whatever destroyed the dinosaurs, they decided to enter cryogenic-sleep or something somewhere to avoid dying. Through a technological mishap, they don't wake up for thousands of millions of years. If humans are still here by then, or if Earth even still exists by then, the aliens come back and want their planet back, war, blah blah blah...

Yay theories...