"Our search for life [has been] based on our assumptions of life as we know it. Weird life and normal life could be intermingled, and filtering out the things we understand about life as we know it from the things we don't understand is tricky."

The tools and experiments researchers use to look for new forms of life - such as those on missions to Mars - would not detect biochemistries different from our own, making it easy for scientists to miss alien life, even if was under their noses.

If aliens from outer space ever come and we show them our civilization and they make fun of it, we should say we were just kidding, that this isn't really our civilization, but a gag we hoped they would like. Then we tell them to come back in twenty years to see our real civilization. After that, we start a crash program of coming up with an impressive new civilization. Either that, or just shoot down the aliens as they're waving good-bye.
A quote from Jack Handy.

The idea of shooting anything different from us does not seem a good strategy to impress alien civilizations. In fact, I cannot see it gaining much popularity at all...
This is perhaps why no contact has been established till now.