With respect to the Fourth Amendment: where does the “reasonable expectation of privacy†arise from?
The 'right to privacy' arises from some liberal unhappy with what the Constitution actually says and trying to invent some right that simply is not there. Our Founding Fathers had the intelligence to realize that we all share this continent and that it would be virtually impossible to live a life totally free of the notice of anyone else, much less an Almighty all-knowing God.
But then of course our Constitution is not a limitation on private individuals. It is a limitation on government. Thus it does not derive rights of the people, but instead places limitations on government acting against those rights.
It is that expectation that is used to justify providing protection for communications that in fact have been fully disclosed to third parties - the phone companies - that have no fiduciary obligation to respect the privacy of those communications. It requires a leap in logic that is not necessarily implied by the words as written.
That's really beautiful and all, but it doesn't have a thing to do with the Fourth Amendment.