We are incapable of having other than a human understanding of anything
We use a human definition of life because we are humans. And because humans made observations that seemed to require a common definition. Some human named that definition "life".
It may be that there are things very much unlike life on earth that exist elsewhere, but that we may wish to extend the definition of "life" to include them as well. Else we will call them something new. Whatever we call them, it will be humans using a human-made definition, that matches human-made observations.