The English “please” is short for “if you please,” “if it pleases you to do this” — it is the same in most European languages (French si il vous plait, Spanish por favor). Its literal meaning is “you are under no obligation to do this.” “Hand me the salt. Not that I am saying that you have to!” This is not true; there is a social obligation, and it would be almost impossible not to comply. But etiquette largely consists of the exchange of polite fictions (to use less polite language, lies). When you ask someone to pass the salt, you are also giving them an order; by attaching the word “please,” you are saying that it is not an order. But, in fact, it is.
In English, “thank you” derives from “think,” it originally meant, “I will remember what you did for me” — which is usually not true either — but in other languages (the Portuguese obrigado is a good example) the standard term follows the form of the English “much obliged” — it actually does mean “I am in your debt.” The French merci is even more graphic: it derives from “mercy,” as in begging for mercy; by saying it you are symbolically placing yourself in your benefactor’s power — since a debtor is, after all, a criminal. Saying “you’re welcome,” or “it’s nothing” (French de rien, Spanish de nada) — the latter has at least the advantage of often being literally true — is a way of reassuring the one to whom one has passed the salt that you are not actually inscribing a debit in your imaginary moral account book. So is saying “my pleasure” — you are saying, “No, actually, it’s a credit, not a debit — you did me a favor because in asking me to pass the salt, you gave me the opportunity to do something I found rewarding in itself!
David Graeber in Debt: The First 5,000 Years (via fivepips)
Words and phrases that have a conventionalized rather than a literal meaning are also known as phatic expressions: other examples are greetings, farewells, and basic checking in such as “how are you?” or “what’s up?”. It’s not that you can’t ask about someone’s actual well-being, but you need to use alternative phrases to do so because certain ones are conventionalized as greetings instead.