Most people seem to think that the universe is full of home worlds for thousands of unique species. But, the truth is, less than 0.1% of the planets the Federation has come across over the last two centuries can support life. Over 99% of all planets in the known galaxy (and likely the entire universe) appear to be uninhabited, and have toxic atmospheres.
Interestingly enough, most sentient species the Federation has discovered, are humanoid, and live on some type of class-M planet (i.e., temperate, tropical, desert, or arctic environments). There are a few exceptions, sure, but only a handful at best. If I were a betting man, I'd say it almost seems like the universe was designed for oxygen-breathing humanoid species.
In fact, there are even some scientists who theorize that all of the sentient species we do come across, were once ancient Humans who chose to leave Earth -- via a gateway similar to the mythical "Iconian Gateway" -- thousands of years ago. Any genetic differences in their DNA were, as they theorize, changed by the God who designed the universe, and chose to separate them from the Humanity they chose to give up.
However these species came to be where we find them, it sure does make our mandate to "seek out new life and new civilizations" all the more exciting. If less than 0.1% of all planets in the galaxy can support life (almost all of these being Class-M), then the goal of our mandate becomes very much like a search for lost treasure. Therefore, if you think about it, as Starfleet personnel, we aren't just deep space explorers, we're treasure hunters as well -- treasure hunters looking for surprisingly similar sentient life, in a galaxy where such life is extremely rare.
I don't know about you, but that certainly puts a whole new perspective on what we do, and why we are out here doing it!
-Lieutenant Sam Archer
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September 9, 2364 -- (Original Devron Timeline)