Of course it is an assumption, but it seems to be a reasonable assumption unless there is specific evidence to the contrary. Much of our understanding of the universe is based on the assumption that we don't occupy a particularly special place in it.

I've heard theories about either the presence of Jupiter, or something about the nature of our moon as providing a safe long term environment for life to form on earth, but even that doesn't feel that special, when there are 10^21 stars with planets in the universe.

Now you are elevating life is common to a default position, simply by declaring it so. I disagree. Me, the complexity of the simplest living thing makes the default assumption that OoL is incredibly unlikely.

You should be aware that the logic you are using, vulgar Copernicanism, was used in the 1700s to conclude that the other planets in our solar system must not only have life, but intelligent life!

What your argument ignores is that we don't live on a randomly selected planet in the universe. We necessarily live on a planet that has life. This "observer selection bias" has to be taken into account; the rarer life is, the more biased our local observations will be.