Thoughts
Smile through it lesson number one:
if you want someone to do something for you, invent a reason that they might want to do it for
themselves. Do not communicate the reason that you want them to do it, or you'll seem selfish.
This is related to Carnegie's principle, "Consider the benefits that person will receive from doing what you suggest" (p. 271), and "try honestly to see things from the other person's point of view" (p. 202) ("why should he or she want to do it?" p. 201).
Intentionally try to hide the fact that you have a selfish motive is not helpful. Leading with a motive that the other people cares about is one thing, but sometimes I've heard people practicing Smile Through It even lie and insist that the reason they give is the real reason when pressed.
"could you move your car so we can get out" is more likely to get someone to move than "have you considered moving your car up there where you're out of the sun."
In most (if not all) of the examples Carnegie gives for these principles, it's obvious that the person has a selfish motive. (One example is person A owes person B money and uses this principle to get themselves more time to pay. Another features literally paying someone to do something.)
But I still don't understand what the difference between using these principal successfully and unsuccessfully is. I also don't understand what has changed in society that makes these principles less effective.