Why do we live on oxygen and not other gases?

In fact we do breathe in other gases besides oxygen but we breathe them out again. For instance, nitrogen which is present in the air around us is inert. This means it will not combine or change anything it comes into contact with. You will be breathing in nitrogen (and other things) with the oxygen but the body needs oxygen.

It does not need nitrogen so the nitrogen leaves the body without altering it in any way. By contrast, oxygen is used by the body first entering the lungs then oxygenating our haemoglobin. It is this enrichment of our blood circulating that enables our bodily system to function properly. We do use nitrogen but not through breathing it. We absorb it through our skin, a different process.