From The Earth Bound Misfit, a gloomy observation (complete with graphic language) that I increasingly share:

As for this, I'm pretty much convinced that humanity has fucked things over past fixing. If you ever saw the beginning to the movie Serenity, it mentioned that humanity fled Earth, pretty much after fucking things up beyond all repair. The reality is that we'll probably fuck up the planetary ecosystem past the point of no return long before we develop the technology to flee the planet.

Which, in a way, may explain why we've not found any hints of intelligent life. For any species that develops to that point probably fouls its own nest and dies off. Just like bacteria in a Petri dish does and just like we're doing.