This is probably something that will be obvious to many but: writing about fictional works but not creating fanfiction/fanon to me tends to fall into four categories, and I think it's really important to get how they intersect before getting weird about other people's posts.

Opinion. Eg: I like relationships based in mutual understanding. This is my opinion. It is correct for me. Might not be correct for you! Note that this is pure opinion, with no argument.

Interpretation: Eg: Beau and Yasha's relationship shows a great deal of understanding in each other's shared experiences, including abusive parents; a shared tendency to believe, at least earlier on, that things will go badly and that they don't deserve better; and difficulty expressing their feelings verbally but letting out frustrations through fighting.

Fact/Evidence: Beau says she is unused to things turning out good, and Yasha believes herself to be, as she says, very unlucky or cursed. Her letter covers that she expresses herself through fighting.

Conclusion: I like Beauyasha.

Argument is Opinion + Interpretation; a valid argument is Opinion + Interpretation + Evidence. Meta can include opinion but ultimately is Interpretation and good meta is Interpretation + Evidence. It's fine to post just an opinion, but if you want to change minds, you need an argument, and if you want to be able to hold your own against a counterargument it better be valid.

You can have the same interpretation and facts and different opinions and therefore different conclusions. Someone else could say "I like messy relationships (opinion) and so Beau and Yasha's shared understanding (interpretation, same as mine) is not interesting to me (conclusion)." You can also have a different interpretation from the same facts, either because you are including a different set of facts on a topic or because there is ambiguity and room for different lenses.

If your goal is to change someone's mind - and to be honest mine usually isn't - you are unlikely to change their opinions. Eg, If someone says "I don't like Beauyasha because I prefer messier relationships" I'm not going to convince them on the grounds of "you shouldn't like messier relationships." I need to either put forward an interpretation that supports a messier relationship, or, more realistically, say "understandable, have a nice day" and move on with my life.

That last bit is important - a lot of people also argue on the basis of "well obviously this interpretation is the only one, so you should draw the same opinions from it." and that's likely to fail; either the person disagreeing with you overall does agree on the interpretation but has different opinions (as in the above example), or they have a different interpretation (eg: Beau and Yasha have attacked each other repeatedly) and that is the basis of their opinion (I don't like that). You need to figure out which is going on and address that should you wish to change their mind. And again, I think you should ask why you want to change their mind.

The last point is that all of the above is valid given the text and reflect different perspectives. But if someone were to say "I don't like Beauyasha because Beau turned into a porcupine and moved to France, and I don't like long-distance relationships" the invalid portion is that this literally did not happen (fact leading to interpretation). If someone can correctly dispute the facts of your interpretation (as they stand at the time, obviously; no one is penalized for not being precognitive) then you are fucked, argument-wise. If I point out that Beau did not turn into a porcupine nor move to France, this person's argument falls apart. They can still dislike the relationship, but if they want to justify it with an argument they better find a new interpretation, and fast.

Before I move into the end I will note that you can also just not vibe with something, and that's purely in the realm of opinion + conclusion, a la "this rubs me the wrong way" or "I find this annoying" or "it's cute" and all of those are valid to hold for yourself, and also inaccessible to anyone else

The conclusions I draw from all of the above are first, making posts that are only your opinion/conclusion is always fine - say what you want - but treating it as an argument is a waste of time, and so anything of the order of "how could you not like xyz (unspoken: because I did)" is pointless because my answer is going to be "easily, and with confidence." Secondly, I think it's valuable to look at posts, even those with arguments, as primarily interpretation rather than opinion. Putting forth a separate interpretation is a disagreement, but it's not a disagreement necessarily directed at you; it's disagreeing with you but the parallel play option is available! You do not need to go out trying to convert all to your same mindset. I think a healthy fandom ecosystem has multiple interpretations and opinions and respectful disagreement; the positive version of "let people like/dislike things" (ie, they can like it or dislike it and if you're normal, your enjoyment is not contingent on theirs). Thirdly, get your facts straight or suffer the consequences. And finally, if you are trying to make an argument for an ongoing work and do so over time, your opinion must be fairly consistent even as the interpretation naturally evolves otherwise it becomes clear that you are arguing in favor of a foregone conclusion (often via moving the goalposts). If you say "I like characters who are willing to make painful and difficult choices, and Blorbo always takes the easy way out, so I don't like them" and Blorbo develops into a character who is able to make painful and difficult choices, if you start critiquing Blorbo now on the basis those choices that's still fair but you better come very correct, and it might be wiser to just say "I don't fuck with Blorbo, if you do that's great but I don't."