The sneakiest boundary-breakers are the polite ones. “Oh come on, just this once” is still a push.

Pressure delivered warmly is still pressure, and it is harder to refuse precisely because nothing looks like a confrontation. A friendly nudge repeated three times is a negotiation you never agreed to enter, and the manners make declining feel like you are being difficult. That is the mechanism, whether or not the person intends it.

Notice how often you are being asked, rather than how nicely. Repeat your answer in the same words without adding new reasons, since new reasons invite more attempts. A pleasant tone works fine on your side; the repetition is what does the work.