system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:Why do people assume that oxygen is needed on other planets to sustain life?

The universe is so vast, isn't there a possibility that "life" on other planets could rely on a different element for survival other than oxygen?
gpt
We don't really assume it, it's just a framework that gives us something to look for. 

Is it possible that there is life out there that is completely different than life as we know it (perhaps silicon based and sustained off methane)? Sure it's hypothetically possible ... but we'd have no idea what to look for, and might not even be aware of it if we saw it. 

So we look for life as **WE** know it, because it gives us criteria to look for rather than cluelessly fumbling around in the dark.

Did that help?