There is a presumption among many people that extraterrestrial life is a foregone conclusion and that it is only awaiting our discovery. The Drake equation is usually highlighted as many astronomers suggest that with the billions of worlds available, that life is all but inevitable1.
Equally there is a sense that life couldn't possibly be unique to a single planet, so there is a strong belief that there must be other planets on which life flourishes. However, we need to consider what the basis for these assumptions actually is.
In the physical sciences, it is clear that the Earth and solar system, represent a localized version of phenomenon that are readily observable in space. Therefore when we look at the laws of physics and chemistry we are reasonably assured that whatever applies on Earth is equally applicable elsewhere in the universe. In addition, the physical sciences are also highly precise in defining the phenomenon being considered, so there is no ambiguity when such scientific laws/principles are being applied.
In biology the situation isn't quite so clear. One of the most obvious problems is that there is still no clear definition of what life is. There are certainly broad definitions and criteria that are presented, but these are fundamentally unsatisfactory and certainly can't be said to apply anywhere except on Earth. This is clearly seen when examining questions regarding whether viruses and prions are alive. More specifically consider that if we discovered a prion or virus on Mars, would we consider it to be proof of life? evidence for life? or just a "dead" organic molecule? The point is that even with such a discovery we still wouldn't have an actual answer to the question of life on other planets2.
Part of the problem is that, unlike the physical sciences, biology has no other environment against which comparisons can be made. The only evidence for life exists on one planet, and everything that has been learned is only applicable to one planet. Biology cannot make the claim that its theories or ideas are universally applicable, since there is no evidence to suggest that life works anywhere else.
More importantly, by not having a formal definition, we have to consider whether biology is necessarily capable of recognizing life on other planets. Are we assuming that extraterrestrial life will be based on DNA and genetics? Do we know if alternate configurations or scenarios are possible?
Despite the billions of other Earth-like worlds that may exist, we also have to consider the problem of how DNA evolved. Do we assume that DNA evolved identically on other planets? If so, that would be extremely fortuitous and suggest that life is a fundamentally singular process, however I suspect that biology won't be that lucky.
In addition, the question of life on other planets is little more than a curiosity question. Like wondering if the guys in the freecreditreport.com commercial are a real band (they aren't). After we know the answer, what difference does it make to anything3? It isn't likely to advance our knowledge of biology on Earth, since it would be impossible to know what events or selection pressures were in existence on another planet. In addition, if the basic configuration of life is appreciably different, then there may be no basis for comparison anyway.
It's probably not worth the risk in bringing extraterrestrial life back to Earth because it could represent unparalleled hazards to our own biology and even if it didn't, we would be artificially modifying the selection pressures on this organism so unless it was confined to the alien environment it would be little more than a "dead" museum piece.
Similarly, to extend the search to include intelligent life, seems quite optimistic given the absence of any encouraging data. I'm not sure why so many people find it difficult to accept the possibility of such uniqueness to life on Earth, or why there is so much resistance to being "alone" in the universe.
I'm not prepared to say that there can be no life on other planets, nor am I prepared to rule out the possibility of intelligent life. However, there is also no basis for the presumption of life being pervasive. Life on this planet wasn't simply the mixing of a few organic molecules, but it was the evolution of microorganisms that "terra-formed" the entire planet making it habitable to the rest of us. In the absence of such activities, it seems that the search for extraterrestrial life is largely going to be a long, tedious, and not a terribly rewarding process.
1 The Drake equation simply assumes that life is simple enough to be reduced to a mere probability of determining how often it occurs. Given our current state of knowledge, any value significantly greater than zero is making some huge assumptions. In addition, it appears that beyond having an Earth-like planet, the moon plays a critical role in ensuring the stability of the Earth at the environmental level thereby avoiding wider variation of conditions for life to evolve and survive.
2 Even our knowledge that viruses hijack the cell for reproduction isn't sufficient if we discovered an alien virus, since we have no way of knowing whether such a situation must occur, or whether it is specific only to Earth and cells here. In other words, viral activity is not a universal law of biology.
3 It's true that discovery of alien life would likely provide new insights into "origin of life" questions, but it is doubtful that it would do much to extend our knowledge of life as it exists on Earth and until we can explore such a foreign world, it can't answer much from that perspective either.
ET: Anybody Home?