There are two types of statements we can make about anything – statements where there is no accepted form of measurement and where there are objective ways of measuring them. Let’s take a few examples. Adjectives like “beautiful”, “nice”, “angry”, “passionate” are generally considered unmeasurable. Therefore, if I say someone is passionate, or a book is good then I shouldn’t have to add an “I think” as it is already implied. However, if I say things like “heavier”, “longer”, etc. as applied to physical things then, since there is a way to objectively measure them, it may be better if I add an “I think”. For example, if I say “my suitcase is heavier than yours” then it may be politer to say “I think my suitcase is heavier than yours”, since there is a possibility that I could be decidedly wrong.
Of course, such rules cannot be applied blindly. While there are ways to measure if my orange is actually sweeter than yours, it is much harder to measure if my daughter is sweeter than yours. According to the above rules, one should say “I think my orange is sweeter than yours”, and just say “my daughter is sweeter than yours”. But, that’s where we would be wrong, since there is also the dimensions of sensitivity and social politeness, and it would be highly recommended that you add a “I think” in the case of your daughter as well, or better still not to say it at all.
Now let’s talk about the murkier area of what is measurable and if measurements can be objective. If you ask a hard core “relativist”, they might say that there are no universal objective truths, and everything we say is contextually decided by who we are or where we belong. While that may be largely true for social statements, some philosophers want to extend it to all truths, including scientific ones. For some strange reason they believe that the only absolute truth is that absolute truth is not possible. Testing the validity of this position is really simple. If one really believes that existence of scientific laws, such as gravity, is just a relative concept which is not necessarily true in all context then all they have to do is to jump out of a high window, and there will certainly be one less person who believes in such ideas.