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.
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Was gonna come here to talk any the great filter. It's basically that there are big road blocks toward becoming interstellar and all species eventually are stopped at one of there roadblocks. The sample size of one, humanity, shows this is pretty likely. Assuming humanity isn't special and assuming we actually do destroy ourselves (we're trying real hard as a species!) then it follows that it's pretty likely that this is a common outcome