I can't remember where I heard this theory, but essentially it goes like this:

If you see an ant on the sidewalk when you're walking down the street, you don't stop to communicate with it, or teach it how to build a television. The theory goes that a civilization with the ability to travel through interstellar space would have a similar knowledge gap to us as we do to ants.

Granted, you also don't stop to ask it if you can build a house on top of its ant hill, so I suppose the analogy works both ways.