Everyone has built up, over their lifetime, an implicit mental notion of "life", which is based upon the sorts of characteristics of things they classify as being alive. Many (I'd say most) people think this mental notion is some objective category that exists external to their mind, some thing that has a kind of essence that is shared by all the instances of life. They see the task of defining life as defining the essence of this category, and see there being an objective question as to whether something (e.g. a kind of virus) fits this category.

But this is to confuse a mental construct with what is out there. There are the objective properties of things like different types of viruses, different types of bacteria, of different types of mammals, and of all other natural phenomena. You can talk about what the similarities and differences are between the objective properties of a type of virus and a type of bacteria, for example, and there are simpler kinds of phenomena vs more complex ones, but there's no actual "life" category out there, separate from our mental constructs, to be objectively defined.