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.
I'm always kind of unconvinced by this one. The universe is unimaginably large and its resources aren't being fully utilized right now. In ecology we generally see that when resources are plentiful, species avoid competition with one another as it's a waste of energy.
The Dark Forest theory is a cool concept for a sci-fi setting or a quick creepypasta, but yeah it doesn't really hold up in real life. If we found out there were aliens chilling on a beach in Alpha Centauri, we're not likely to pack up our shit and sail into the lifeless void just to punch them in the dick for the hell of it.