Human beings are obsessed with the possibility of life on other planets. Countless movies, books, and television shows are centered around alien civilizations, and there is an entire subculture of people devoted to the idea that aliens not only exist, but have already visited our planet for the inexplicable purpose of livestock mutilation and the molestation of the barely-literate.
However, we can say with a candor close to absolute certainty that nobody on Earth will ever see warp drive starships or quad-breasted space women from the Crab Nebula, because for a number of reasons, the chances of us ever meeting any aliens are slim to none. Why? Well ...
5 If They Exist, They're Likely Too, Well, Alien
Our concept of alien life is firmly rooted in the idea that it would be comparable to life on Earth. Almost every major alien race in the realm of science fiction and fantasy has been humanoid -- the Klingons from Star Trek are essentially angry space-cavemen while the Vulcans are virtually identical to human beings. Both Luke Skywalker and Han Solo are technically aliens, despite the fact that they look exactly like two white Earth men from the 1970s, and the Asari from the Mass Effect series look like blue strippers with subdermal cornrows.
Behold, inspiration for decades of uncomfortable Comic-Con costumes.
Even the so-called "eyewitness" accounts of extraterrestrial landings we see on those History Channel UFO specials describe the aliens as being essentially human in appearance, with identifiable arms, legs, eyes, and heads. Hell, even the xenomorphs in the Alien franchise, which were supposed to be inhuman bug monsters, were so close to us they could be played by humans in costumes. The point is, that our idea of intelligent life is dependent upon the assumption that the human race is a common point of reference, despite the fact that in all likelihood this is crushingly incorrect.
But the greater question is, "If aliens aren't going to have boobs, do we even want to meet them?"
The truth is, we don't know the first goddamned thing about what the makeup of life is in the Quasar Nebula or whatever distant star cluster aliens might hail from. The dominant life form on their planet could be a fart cloud of neon gas and space lightning that communicates with a series of atonal whistles, by changing the temperature of the air around them, or even by emitting certain smells (though to be fair, some human beings communicate in this third fashion).
In other words, even the basics of life as we know it on Earth (I.e., that carbon is the base element) is by no means the fundamental rule of the universe. Carbon may not even freaking exist on the aliens' home world. The reason we have yet to make contact with alien civilizations may very well be because we wouldn't recognize them even if they sat down in the booth next to us at Applebee's.
"Sorry, we don't serve hyper-intelligent shades of blue. Store policy."
But even if they were somehow amazingly close to humans in their psychology, their thought process could be significantly different to human in terms of speed and response (an idea postulated by Carl Sagan in the novel-turned-unfortunate-movie Contact). It could conceivably take an alien species several hours, days, months, or even decades to relate a single phrase translatable by human beings. For all we know, aliens have been paging us the same sentence for the past 70 years and we've just been dismissing it as bizarrely drawn-out radio static.