Q: If there is no agreed upon definition of literature, how and why should we give our own views with rationale and justification?

Your question isn’t clear. Rather, it’s not clear what you are seeking help with. Are you doing a study in literature or the humanities? Are you writing a literature review? Are you doing a study in which you need to provide the rationale/justification? Or are you a doing a views piece?

As the intention isn’t clear, we’ll reply in short to each of the above points/questions.

• Yes, works and studies in literature and the humanities are often open to interpretation, but that’s a given in the field. That’s why studies and papers in these fields are often critiques or analyses, presenting one’s learned perspective on a subject. (Alternatively, they involve more of qualitative research.) Speaking of which, there are also papers in the ‘hard sciences’ that can be about presenting one’s perspective. In both areas though, you need to back your perspective with data of some sort.
• If you are doing a literature review or some sort of secondary research, you may refer to this all-inclusive piece talking about the various kinds of secondary research you could pursue: Secondary research – the basics of narrative reviews, systematic reviews, and meta-analysis
• The rationale or justification of a study is the reason why you are conducting the study. This is not so much your personal motivation, but the kind of knowledge you wish to add toward the topic. You may learn more about rationale/justification here: How to write the rationale for research?
• A views piece, also known variously as a perspective / opinion / commentary / think piece offers a critical analysis or opinion of a particular topic. It may present your viewpoint or worldview, but it has be supported by strong reason, often in the form of data. You may learn more about perspective pieces here: A young researcher's guide to perspective, commentary, and opinion articles

In case you are looking for help with some other topic or matter related to scientific / scholarly / academic publishing, you may look up the site using the relevant keywords.

Hope that helps!