My theory is that in order for an extra-terrestrial race or civilization to have evolved to the point of inter-stellar travel, they would have had to have learned to evolve their consciousness beyond the stage of primitive warfare and destruction. Use our current civilization as a starting point. Our technology threatens our very existence as we still operate in the mindset of 'waring tribes'. We run the risk of annihilating ourselves with our combination of primitive consciousness and our advancing technology. Other worldly civilizations would have destroyed themselves had they not learned to evolve beyond this approach to 'existing'.

Now one might suggest that an advanced civilization could easily wipe out a distant civilization with more primitive technology. But why would they do that? Once you have developed the technological means for inter-stellar travel, can't you essentially go anywhere you want in the Universe for what you need/require? It would be like killing another man on the beach to steal the sand in his pocket. There is sand everywhere, you do not need to kill that man for the sand in his pocket when it can be found in abundance all around you.

Additionally, taking a hostile approach to interaction with distant civilizations is a form of mutually assured destruction. If you employ a hostile approach with your neighbors then you are essentially inviting equal treatment towards your own civilization. Portray yourself as a 'threat' and you will be received as such by the rest of the 'community'. There is always a smarter, bigger, stronger 'badass' to come along and put you in your place. Your own survival is contingent on how you conduct yourself. Seek destruction with your actions and you will meet the same fate. Can't get any more straightforward than that. : )