Oh, come on, you call that saccharine? This is saccharine XD

As for insincerity, it's not that I'm insincere, it's that I'm very profuse in my demonstrations of affection, respect, and so on. The emotion behind is genuine, I'm just very openly demonstrative about it. If the other person has codes where only a much greater favour is deserving of such open displays of positivity, they see the discrepancy and deduce that I am sending false (or exaggerated) signals.

The same is true in reverse. Some people have a norm of violently lashing out at anything they dislike, and, when meeting people who react to offence by silence or by shifting their attention elsewhere, some very unfortunate misunderstandings can happen.

Essentially, it's a misunderstanding.

Thus, if many people behave in that way, I would be forced to choose between being rude and being insincere.

I don't think it's insincere if you contain your attempt at rudeness and yet endeavour to convey your misgivings and negative feelings to the other person clearly and sincerely. To take an exaggeratedly dramatic example, when Iñigo Montoya says "Hello. My name is Iñigo Montoya. You killed my father. Perpare to die.", he's being perfectly sincere. More so, in fact, than if he'd said "YOU FATHER-MURDERING PIECE OF SHIT, I'LL SWORDFUCK YOU TILL YOU LOVE IT!", because it's more precise and accurate. Even though his first impulse might have been to say the latter rather than the former (we know it wasn't, but that's beside the point).

Another cartoony but very nice example of someone very angry still being polite about it, and not in a passive-aggressive or insincere way, but in an open and assertive way, is Finn the Human from Adventure Time. There's plenty of force behind "NO, MAN" or "GET OUT" without having to intercalate "fucking" and "asshole" and so on inbetween.