Great Filter Theory
The "Great Filter Theory" May be one reason why we haven't met alien life yet as described in this Topic: Directly Meeting With Aliens. The Great Filter Theory supposes that beings can only evolve to a certain point and then die off. You may think that isn't possible because smarter you are the wiser your choices. Not so, its very easy for our primitive technology to wipe us out now (Nuclear weapons) and we've already used them, many times.