While one can obviously claim that we are the only known living things
in the Universe based on the scientific evidence, it is also quite obvious
that we have explored blessed little of the Universe to make any kind of
serious declaration in that matter.

I am sure a number of people on this planet don't want there to be
intelligent life elsewhere because of the fear that it would be superior
to us in some way - and we all know what happens to a lesser culture
when a superior one (or at least one with bigger guns) comes along.

There is also a fundamentalist religion factor that plays into this.
One of the big proponents of the Galactic Habitable Zone - which
wouldn't ya know it just happens to be right where we are and we
alone - Guillermo Gonzalez, who is a fundamentalist Christian with
a definite bent to make sure nobody else exists in the Cosmos.

http://www.pandasthumb.org/archives/2004/07/the_privileged_1.html