The recent news about a new supermassive virus being discovered got me thinking. What biological differences between viruses and cellular organisms have made viruses be deemed non-living?
If this is a topic that really interests you, I'd suggest searching for papers/reviews/opinions written by Didier Raoult. Raoult is one of the original discoverers of the massive Mimivirus and his work will lead you to some truly fascinating discussions that I couldn't hope to reproduce here.
The main argument for why viruses aren't living is basically what has been said already. Viruses are obligate parasites, and while plenty of parasites are indeed living what sets viruses apart is that they always rely on the host for the machinery with which to replicate. A parasitic worm may need the host to survive, using the host as a source for energy, but the worm produces and synthesizes its own proteins using its own ribosomes and associated complexes.
That's basically what it boils down to. No ribosomes? Not living. One advantage of this definition, for example, is that it is a positive selection (everyone alive has got ribosomes) and eliminates things like mitochondria that are sort of near the boundary of other definitions. There are examples on either side of something that breaks every other rule but not this one. A common one is metabolism and while that suffices for most cases some living parasites have lost metabolic activity, relying on their host for energy.
However (and this is the really interesting part) even the ribosome definition is a bit shaky, especially as viruses have been found encoding things like their own tRNAs. Here are a few points to think about:
I'll wind down here, but suffice to say that while our current definition may have sufficed for a while, and still does, it is no longer so solid. In particular, there is a theory alluded to above that eukaryotic life itself actually formed because of viruses. I can expand on this if you like, but here are some great sources:
Boyer, M., Yutin, N., Pagnier, I., et al. 2009. Giant Marseillevirus highlights the role of amoebae as a melting pot in emergence of chimeric microorganisms. PNAS. 106(51):21848-21853 (http://dx.doi.org/10.1073/pnas.0911354106)
Claverie, JM. Viruses take center stage in cellular evolution. 2006. Genome Biology. 7:110. (http://dx.doi.org/10.1186/gb-2006-7-6-110)
Ogata, H., Ray, J., Toyoda, K., et al. 2011. Two new subfamilies of DNA mismatch repair proteins (MutS) specifically abundant in the marine environment. The ISME Journal. 5:1143-1151 (http://dx.doi.org/10.1038/ismej.2010.210)
Raoult, D. and Forterre, P. 2008. Redefining viruses: lessons from Mimivirus. Nature Reviews Microbiology. 6:315-319. (http://dx.doi.org/10.1038/nrmicro1858)
Scola, B., Desnues, C., Pagnier, I., et al. The virophage as a unique parasite of the giant mimivirus. 2008. Nature. 455:100-104 (http://dx.doi.org/10.1038/nature07218)
I agree with the answers already given, these are the reasons that viruses are not considered alive. I want to point out though that this isn't an area you find 100% agreement on; there is a decent subset of biologists who do consider viruses alive. I would say - completely on the basis of personal observation - that virologists themselves are the group most likely to claim that viruses are alive.
There are quite some different definitions of being "alive", but a common one includes the need to have responsiveness, growth, metabolism, energy transformation, and reproduction (found from the Encyclopedia Britannica). Viruses depend on host cells to do all this, so seen alone as a virus outside a host cell, they are not alive.
There's another short, but to the point blog entry about this.
It is only a question of definition. You can set the boundaries between living things and not living things anywhere.
Some philosophers have argued that using a clear boundary between living and non-living things is not such a good solution. In nature there would rather be a continuum from a stone to a bacteria.
It is true that in thinking of viruses such as Lausannevirus or Marseillevirus we might be willing to integrate them in the category of living things. These viruses are giant, and even can be parasitized by other viruses.
Viruses are made of proteins and contain nucleic acids (RNA or DNA). If you consider that they are alive, what would you say about viroids? A viroid is just an nucleic acid that is able to infect a host and cause the replication of itself. What about a prion? A prion is a protein that, roughly speaking, has the same consequences as that of a viroid.
I think (one should check the literature, I might be mistaken) that there is a species of parasitoid wasp that produce out of its own genomes, viruses that reduce the host immune system in order to make the caterpillar a suitable habitat for the egg. Is this virus alive? Isn't it just a toxin of the wasp?
I guess one reason of considering viruses as non-living is that we do not know how to branch them in the tree of life ! Some might argue by the way that viruses would not at all form a monophyletic group.
There are several people tackling the question of "what is alive". Unfortunately the best book I know on the subject comes from the French literature; it is called "Comment définir la vie ?" by Bersini and Reisse. In this field, the most popular authors are Varella and Maturana. Again, if I'm not mistaken, the definitions of life are quite different from philosophers, from people having interest in the origin of life, and from people seeking a definition suitable for extra-terrestrial life.
Despite great answers from Amory and Remi.b, I want to emphasize this: there is continual debate about the definition of life because "life" is not something that exists in the real world.
People seek a definition of life that satisfies an intuitive notion of what alive should mean. They feel that, say, intracellular parasites should be considered alive, but (say) only if they have an enclosing membrane, like Rickettsia, and not if they are just a virus, or just an RNA molecule like a viroid.
While people have a built-in intuition that, for example, reliably categorizes a tiger as alive and a rock as not alive, that intuition can't be precisely bounded by a definition such that everyone is satisfied by the boundary. There are arrangements of matter in the physical world that fall outside the clear region of the intuitive concept of life, and this leads to continual, unresolvable, argument on what a precise definition of life should be, and what it should include.
In addition to the good answers given here, I would like to propose a more intuitive argument against viruses being alive.
Viruses are, at one point of their "life", simply a piece of DNA (or RNA). Would you consider a piece of DNA to be alive? If so, then are transposons alive? Are chromosomes alive? How about synthesizing a piece of DNA - is that creating life? The answer will probably be "no" for most people.