This question is based on the assumption that intelligent alien civilations must be like us, with a desire to go out and 'colonize' the entire galaxy or universe with no concern for other life that might already be there, or could potentially arise. The fact that we don't see any of those galactic colonizers with their photon torpedos aimed at us from giant military fleets ready to conquer and destroy just means there aren't any (or they haven't got here yet ...
). It doesn't mean intelligent species aren't abundant. It could mean that the universe filters out those who would desire to go and 'colonize' other planets. Species with this attitude just don't make it. How has that colonizing attitude worked out for us so far? Depends on who you ask I suppose, hmmm, how about asking the native north americans, australians, africans ..., or all the species of animals and plants we are currently eliminating, or the planet itself, you know, our home. Only a bunch of complete idiots would destroy their entire planet and dream about doing the same thing over and over and over again to all the other planets in the universe, and then wonder why nobody else is here doing the same thing to us. Then we declare since 'they' aren't here colonizing us, they don't exist, or call it a paradox and endlessly debate it! Yup, I'm convinced, we are alone!