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.
4 They Might Not Want to Screw Us Up
But let's say that there are aliens out there who, by some random chance, evolved to be close enough to us that we could converse with them. And let's further assume that they don't simply want to destroy the planet and take our gold. They may still have solid reasons for simply not wanting to talk to us.
You can only leave so many crop circles without a reply before you start looking desperate.
If you've ever seen Star Trek (and let's not kid ourselves -- if you clicked on this article, you have seen Star Trek), you may know that the most important rule the crew of the Enterprise follows as they're zipping around the galaxy is the Prime Directive, which they break about eight times an episode:
"As the right of each sentient species to live in accordance with its normal cultural evolution is considered sacred, no Star Fleet personnel may interfere with the normal and healthy development of alien life and culture. Such interference includes introducing superior knowledge, strength, or technology to a world whose society is incapable of handling such advantages wisely … This directive takes precedence over any and all other considerations and carries with it the highest moral obligation."
"And that means no banging! Kirk!"
Basically, the Enterprise cannot interfere with life on other planets in any way, regardless of the outcome. They can't pick one side of a warring alien race and arm them with laser cannons, obviously, but they're also not allowed to bring a starving village a crate of space cupcakes (Starfleet considers these to be the same goddamned thing). They have literally let entire civilizations obliterate themselves for the sake of the Prime Directive. This is an actual theory about extraterrestrial life that scientists call the "Zoo hypothesis", which suggests that super advanced aliens do exist, but they refrain from contacting Earth in order to keep from interrupting our natural evolution and development. Essentially, the aliens are content to sit back and watch until we reach their level, and only then will they share their time-traveling rocket boot technology with us.
What a bunch of shitheads, right? Well, maybe not. Charity, no matter how well-intentioned, can backfire in unpredictable ways. For example, there have been a number of projects to increase the standard of living in Ethiopia, which is a country literally famous for being impoverished. One such project involved installing water taps in Ethiopian households. While this had several incredibly beneficial effects, most notably dropping the infant mortality rate, it ultimately contributed to household shortages. More children means more family members to support, and those water taps weren't spitting out any extra money -- so essentially, the children were surviving infancy only to grow up starving. By solving one problem, another equally devastating problem was created.
"I'm finally over my intimacy issues, but now I have herpes."
In the perspective of the Zoo hypothesis, imagine if aliens had touched down on Earth during the Dark Ages, and instead of face-raping everyone with space crabs, they decided to try and help. So they gave humanity self-perpetuating mooncube energy and advanced irrigation, thereby eliminating the need for any of us to ever compete for resources ever again. The aliens leave, patting themselves on the back for their good deed. However, with endlessly renewable resources, Earth quickly becomes overpopulated, and instead of fighting for oil or whatever, people are vaporizing each other with mooncube lasers just for living space. The aliens come back after a hundred years to check up on us only to find the Earth an ashen, blasted landscape of corpse fumes, and now they have to type up a substantial report explaining their actions to the intergalactic community.
We wouldn't want to have to deal with that amount of paperwork, so it's reasonable to assume that the aliens wouldn't, either. But even if they did, there might be another reason they avoid us ...