"The Framers never for a moment thought the president needed a Mondale-like adviser or a Cheney-like super-deputy. Their main concern was that they wanted electors from the states to be forced to vote for two people, and not from the same state. The reasoning, historians surmise, is that states would habitually throw their support behind a favorite son as the presidential candidate. Virginians would vote for a Virginian, New Yorkers for a New Yorker, etc. But if they had to cast a second ballot, that second choice, under the Constitution, couldn't be another favorite son.
Follow this logic to its conclusion: The Framers were thinking that the No. 2 pick of many of the electors would be a nationally recognized figure who would wind up with more votes, total, than any of the No. 1 picks. It's kind of like they wanted the vice president to be president."
Makes sense--in today's world the idea of a "favorite son" has faded, but that was a real fear in 1787.