Mine is probably the most boring: There are many intelligent species in the universe. Faster-than-light travel, however, really is simply impossible, meaning that there cannot exist a truly interstellar civilization. So while some species have probably settled solar systems other than their own through generation ships, suspended animation, time dilation, or whatever, their range of expansion is limited. This means that encounters between species of different planets are rare. Humans will most likely never contact any intelligent alien species, at most one or two. We might, however, discover evidence of their existence through telescopes or something.
Reverse Posadism: All the aliens are trapped in capitalist/feudal/fascist hell holes and we must be the socialist aliens to liberate them.
Mostly joking. Assuming FTL travel is real and possible, I don't think it's that hard to imagine that whatever community would exist out there, whether one species or a collective of species, would have a sort of prime directive-style rule which would exclude us from interacting with them. It's not hard to imagine what kind of freakish hell capitalists would turn earth into if they got their hands on tech far beyond our own. But that's just me hoping that there are Trek-like communist aliens living the dream while I'm stuck watching the planet die. :doomer: