The prime directive! I don't know how much more obvious it could be. If a species was advanced enough to reach us, the chances they are benevolent is very high (though not guaranteed) because otherwise their likelihood of self-destruction is just too high. And if they are that advanced, they would probably not want to recklessly interfere with the development of another species. There are a lot of New Age theories about this, I don't fully buy the specifics of any of them but in general this situation has always seemed extremely likely to me and it was right out there for me to ponder since I was a kid watching Star Trek.
I also think we are too stuck in our perspective to see many other likelihoods. We assume that we have a pretty solid definition of life, but we really have no way of disproving Gaia theory, who is to say that planets themselves are not forms of intelligent life. Who is to say there isn't life we are unaware of in different realms, if aliens visited us, we might not even be able to perceive them. Once again, lots of conspiracy theories that talk about this, no need to put full faith in the details but it seems pretty likely once you realize that humans always seem to think they know much more than they do.
If there are intelligent species out there, and it seems likely that there are more than one, it the most sense that they are patiently watching us, either hoping to make contact and help us advance when we are ready to do so peacefully, or ready to shoot us down when we become a threat, probably both.
yes this is an plausible observation, indeed life can probably be vastly different to what we are, but even if they're cant, there should be ample opportunities for the galaxy to be full of intelligent beings similar to us.
To assume that we are intelligent enough to handle being part of an intergalactic community is a very human perspective. :-D The one's like us are all probably in situations similar to us, not yet able to travel to us or in the process of self-destruction. If we can learn to stop looking for enemies and create a culture where love and inner peace guides our decision making, the ETs will appear, I promise.