SF author Robert Sawyer was recently asked a question about this: "What's your opinion of Fermi Paradox?"
Sawyer's reply: "Enrico Fermi raised a very valid point: if the universe should be teeming with life, then where are all the aliens? My fear is that terrorism ultimately destroys all civilizations: that is, as more and more destructive power ends up in the hands of individuals, someone eventually does enough damage to destroy each civilization."
Being friends with Sawyer I chimed in with:
"Why haven't we encountered aliens or at least evidence of them? Depending on who you ask maybe we already have. More seriously I wonder if whether we're facing a variation of the rare Earth theory. Life might indeed be common in terms of simple or non complex life like on the microbial level or such. And there might be worlds teaming with complex life on the animal level like Earth of the past before the rise of human intelligence and human civilization. The rarity mightn't be in life or even a measure of intelligence, but in intelligence giving rise to advanced technological civilizations. It could happen, but given the rarity and average distance between star systems with advanced civilizations the odds could really go up in terms of them noticing us, coming here or us noticing their presence either nearby or from a distance. Anyone looking in our direction from beyond 100-200 light years distant might likely never notice a sign of us being here, given we've been technologically "visible" or "noisy" for barely a century. Also beyond a few light years our star system probably doesn't look all that exceptional. From our position we might not see evidence of alien civilizations if they are distant enough and if they haven't been technologically "visible" much longer than we have. Another point---and this may be speculative (hell, it's all speculative)---perhaps a truly advanced civilization no longer uses the same forms of readily recognizable communications that we're so familiar with. Imagine ancient man who couldn't envision the possibility or existence of something like the internet, radio, a telephone or the telegraph."
Thoughts anyone on where is ET?