Under the original Constitution electors didn't cast votes for president and vice president, they each cast 2 votes for president. Whoever got a majority won, and the runner up became vice president. The original rule was each elector could only cast one vote for someone from his state. The other vote had to go to someone from another state. The idea was to prevent a situation where the electors all choose candidates from their own states and no one gets a majority or reasonable plurality, throwing the system into chaos. Our national political identity was very underdeveloped in the 1780s, so that scenario wasn't as crazy as it might sound today. They hoped that by imposing this rule it would free up one set of votes to go to a candidate with national appeal, even if a favorite son got everyone's first vote. After Aaron Burr tried to steal the presidency from Thomas Jefferson the 12th amendment changed it to the current system where each elector casts a vote for president and a vote for vice president, but they kept the rule that required that at least one of the votes go to someone from outside the elector's state.