Kindness requires sincerity. It comes from the heart. Politeness can be compelled by forcing you to follow agreed-upon rules. Knowing the distinction will help you avoid disappointment in life. This is about the power of genuine heartfelt gestures.
Agree with you that spontaneity is the key to seeing the difference between being polite versus being kind.
Reminds me of Malcom Forbes' quote: "The true measure of an individual is how he treats a person who can do him absolutely no good."
Great, subtle, analysis!