It's one of my personal pet peeves, while to many people, it is a favorite and (so it seems) very enjoyable pastime: Dishing out truths, mostly in the form of advice, without any reasoning or evidence to back up their claims.
It wouldn't even bear mentioning, given that, well, people are just people and will keep enjoying this kind of interaction... except that I have the impression that many people honestly have no idea of concepts such as levels of truth, or how to back up a claim with arguments, or how to phrase stuff properly. It's not that they're dumb, they simply never had their claims challenged - probably because nobody takes them seriously anyway.
So here's the deal:
For one, I think it is important to signal whether you consider your claim to be subjective or objective; a universal, absolute truth, or just an adhoc explanation for an observation you made. That's why you will frequently see me introduce a sentence with the phrase "I think" in this blog. It signals that this is a subjective opinion for which I can not necessarily provide any evidence.
I think that there are three levels of "provenness" to a claim: Let's call them "none", "by argument" and "by evidence". I am not arguing that you shouldn't have opinions without evidence - that is clearly impossible - but I do argue that we should be aware of that distinction, and we should somehow express it in our language. Of course, this is a very crude and broad distinction - you can specify numerous sub-categories if you like. Well, it's a start, nothing more.
Many of our opinions are indeed made out of thin air; maybe we just copied them from our parents without giving it any thoughts - and, while this may not be ideal, it is unavoidable and therefore totally okay. They're not proven, except perhaps by authority. "Red lips are beautiful" is an example of that. There isn't a lot to say about that - you can agree or disagree, and that's about it.
The next category, which I call "by argument", does not have any real evidence attached. For example, "I think that red lips are beautiful because they signal sexual availability." If you contrast this to the previous version, you can see that this time, you can at least argue whether that's what red lips actually do. You can start a debate.
Of course, if you want to phrase a real argument, you need to provide evidence on top of reasoning. "I think that red lips are beautiful because they signal sexual availability. That book that I read has a lot of examples of that." Now I can look up that book, read it and make up my own mind. It makes sense to look for counter-examples or challenge the reasoning. And that's what makes these kinds of claims so much stronger than the other ones: They are open to challenge on many levels, and therefore allow us to actually learn something about the world.