In the case that in this world, being one of many possible worlds, life may have only began on this planet. Out of the infinite number of possible planets, within an infinite number of possible worlds, we may be in the world where life is only on one planet. What reason have we to assume that life MUST be on other planets just because it is on ours? Just because a planet has life, does not posit that others must or should as well. If we are a product of chance, then it is simply that and also explains why there are not others.
The aliens killed themselves in a nuclear war.I am not kidding you, I have heard such explanations quite often for why we still haven't made contact yet.
The Fermi Paradox backfires, because it is a better support for agnosticism than it is for faith or believe in theism. Basically, it puts God and extraterrestrial into one category of unidentified objets. http://www.reasonablefaith.org/forums/choose-your-own-topic/what-god-does-an-alien-could-too-6024825.0.html
I find the Fermi paradox a good argument against a galaxy filled with intelligent life (and also future time travel development). It is also interesting that a former head of SETI thinks it unlikely there are intelligent aliens out there, and his books are worth reading (Paul Davies).However, the best book in making the evolutionary case that human-like creatures were inevitable given evolutionary laws and the initial set-up of the universe, but that also at the same time the odds against them developing were so astronomical as to make it highly likely that we're the only instance of human-like life in the universe, is Simon Conway-Morris' book: Inevitable Humans in a Lonely Universe.
Today the pendulum has swung dramatically, and many distinguished scientists claim that life will almost inevitably arise in Earthlike conditions.
On the other hand, if life arose simply by the accumulation of many specific chemical accidents in one place, it is easy to imagine that only one in, say, a trillion trillion habitable planets would ever host such a dream run. Set against a number that big — and once you decide a series of unlikely accidents is behind the creation of life, you get enormous odds very easily — it is irrelevant whether the Milky Way contains 40 billion habitable planets or just a handful. Forty billion makes hardly a dent in a trillion trillion.
I saw a picture a few years ago of some primitive tribe somewhere (Brazil?) that had never had outside contact. The picture was taken from a plane or helicopter and shows some women and children and a man aiming a bow and arrow. I don't know if there are any undiscovered human tribes in existence today on the earth, but I suppose there might be.Compared to the earth, the universe is huge so I'd don't feel Fermi's paradox is very convincing.Another thought is that aliens might be so advanced that they don't want to bother with us at our present stage of development.
Another problem is the feasibility of space travel itself. We do believe that more efficient means will be invented, but for now the perspectives are bleak. Visiting the three nearest stars with the speed of light (of which we can attain only a small fraction right now) in one ship would take thirty years. Visiting one hundred stars would take enormous effort and resources. And for what? People are curious, but not that curious. Reaching farther stars would require colonization of intermediate systems, perspectives of which are even bleaker...Finally, life has appeared on Earth relatively fast, so we do have reasons to believe it should be rather common. However, we cannot say the same about intelligent life - this might just be a fluke. We might as well wonder whether any life-friendly planet would produce anteaters - well, theoretically, it might, but it is far from certain.