Sure, no problems, whatever you want

We don't mean what we say, nor do we say what we mean.

That is a basic feature of communication that is usually unnoticed. It is related to the fact that there is no unique definition of the concepts we use in language, and to the way we communicate on many layers all at once. Nothing we say simply conveys a message relating to the context of the sentence we speak, but it always includes something of our mental condition at the moment in which it is uttered, the relation to the listener and other features of the context.
For instance, say you wish to borrow a book of mine. Albeit my lack of attachment to private property, it is possible that you know that regarding my books I display all those primordial tendencies that possession can arouse. Say the conversation goes like this:

"Can you borrow your copy of Hitchhiker's Guide to the Galaxy?"
"Sure, just make sure you bring it back."
"Oh yeah, no problem. Of course I will bring it back."
"Great, then you can have it."
"Thanks. (pause) You're sure it's alright?"
"Yeah, sure. (pause) Just make sure you bring it back."

I take it that you agree that this conversation seems reasonable, though if we only said what we mean and meant what we say, it would probably take the following form:

"Can I borrow your copy of HHGTTG?"
"Yes, provided you give it back."
End of conversation.

The second conversation covers just the same amount of 'meaning', conceived in a narrow sense of references, as the first one. However, the second conversation misses the richness of communication encapsulated in the seemingly redundant repetition of the first, likely conversation. In addition to asking the book from me, you want to know my attitude towards the book or books generally, as to make sure that I mean what I say when I say you can have it. You would be happiest if I just replied enthusiastically "Sure! You can have it for as long as you want!", which would seem strange despite the joy I take in the spreading of a good piece of writing. I, on the other hand, would like to convey the message that I am happy to lend the book provided you give it back. I don't want to make a big deal out of it, but I do want to make sure that you know that even though I have not opened this book for years I would really be upset if you failed to give it back and it would be missing from my library.
Making sure we understand each other is an important feature of communication.

Ironically, it usually entails repeating points we have already made rather than making new ones.