Here's a thought-provoking article by Andrew Snyder-Beattie of the Future of Humanity Institute at Oxford University. In it, the author runs through the Fermi Paradox, which in a nutshell goes something like this: given the age of the Universe and hence the amount of time for sophisticated technological civilisations to arise, why haven't we seen evidence for them yet? If life and the subsequent rise of technological civilisation is common, then surely the galaxy should be teeming with them by now and they should have had enough time to develop interstellar communication and travel...
One possibility is that life may be unique to Earth, and we really are alone in the galaxy. However, every new discovery of a habitable exoplanet (such as Kepler-186f announced last week) slightly reduces the chances of this. Another unsettling possibility is that life and civilisation has arisen on many other planets, but goes extinct before becoming advanced enough to communicate between stellar systems.
Admittedly, I haven't given the Fermi Paradox too much thought, but it seems a bit dodgy to me. To start with, the only serious searches made for extraterrestrial civilisations have been aimed at detecting radio signals from a relatively tiny number of stars. But even if there were advanced civilisations around each of those stars, why would we expect them to be blasting radio waves out into space in large enough quantities to be detected by our humble telescopes over vast interstellar distances? Seems like it would be a massive waste of energy to me. I might be wrong on this, but human civilisation is relatively radio quiet these days from the vantage point of an extraterrestrial astronomer - there was a brief period of a few decades last century when our radio technology was so primitive that there was a lot of loss to space, but this is greatly reduced these days thanks to more efficient transmitters.
On the other hand, why haven't we been physically visited by aliens in spaceships? Again, I'm not sure this is too surprising, even if loads of space-travelling aliens are out there. Supposing that civilisations arise on 1 out of every 100 habitable exoplanets (perhaps advanced lifeforms emerge on the 99 others, but never evolve a species with human-like intelligence), that would mean many 1000s of civilisations in the Milky Way alone. The only unavoidable reason for them to move to a new stellar system (e.g. ours) though would be if they desperately ran out of resources at home or their star had reached the end of its life. But assuming they chose their new home randomly, there'd only be a 1 in 100 chance they'd choose Earth.
This is all very hand-waivy, but I personally don't think the so-called Fermi Paradox is necessarily that paradoxical. Anyway - a great article by Snyder-Beattie, who I'm sure has pondered it longer and harder than I have!
While emergence of intelligent life could be rare, the silence could also be the result of intelligent life emerging frequently but subsequently failing to survive for long.