Well what is the point of anything, if all you can say is "I'm right because I think I'm right"? That's just an arbitrary judgment. Would anyone be convinced by that argument?
Let me put it this way.
We know that a "particular kind of a smile (Te smile)" is something that objectively exist "out there" in the real world. But when you're the one that is observing it, then it's only a partial observation. How do we know that that smile doesn't exist outside of your observation? For example, others might point out that other types have the same smile. So when you claim, "Well I see it in Te types! It must be a Te thing!", that's not very convincing.