Sometimes I wonder if the reason we've not found any life might be because we just don't recognise it as such. It might be too alien; maybe it's such a large system that we don't fully comprehend it, or perhaps it moves at a timescale that we just cannot grasp.
We created gods in our image to explain existence. Anthropocentric as we are, we assumed that humanity was somehow special, distinguished from all other life on Earth. Now we're doing the same with the very definition of life. Life looks a certain way on Earth, so obviously it needs to look the same everywhere.
It makes sense as an outset though, you can only look for what we know to look for.