The major cause of conflict between cultures is competition for resources, and it seems to me that all space-faring species will only require two resources: matter and energy, both of which abound throughout the universe. The only thing which can be realistically traded between separate species is information, knowledge. All other commodities would be easier manufactured at home than having to go off-planet for them; either aquisition by trade or conquest. Even the most belligerent and hostile species would tend to gain more by peaceful data exchange than by direct conflict. The probable separation between intelligent species (on the order of several hundred to several thousand light years) would make direct contact highly unlikely unless Einstein got it all wrong. I suspect Einstein got it right or THEY would already be here.

Having said that...what can't be ruled out is simple paranoia. A sufficiently advanced species, dispersed throughout many star systems, would be effectively immortal. It could only have one possible threat to it's existence: another advanced, spacefaring species. It's not the aggressive, hostile civilizations we have to worry about, it's the peaceful meek ones, which might think it wise to eliminate all potential competition before it gets strong enough to be dangerous. The high risks, certain costs, and probable impossibility of interstellar warfare make it likely that ET will be very eager to find out if it has neighbors, but will probably do everything it can to avoid giving away its location. Everybody out there is listening as hard as they can, but they're going way out of their way to keep quiet. I think that would also be a prudent strategy for us.