If we should pick up signals from alien civilizations, Stephen Hawking, our century's Einstein, warns: "we should have be wary of answering back, until we have evolved" a bit further. Meeting a more advanced civilization, at our present stage,' Hawking says "might be a bit like the original inhabitants of America meeting Columbus. I don't think they were better off for it."
Mankind has always been driven by contradictory drives. The relentless curiosity that pushes us forward and is directly responsible for our progress from caves to cities. The fear of change that tells us "hang on, these caves/cities are really nice, we don't want to risk losing them." There isn't any greater potential threat to the status quo than the discovery of extraterrestrial life, which is why some people would prefer we didn't try.