Why is there a twelfth amendment?

Originally the person with the largest number of votes in the Electoral College became President and the person with the next largest number of votes became Vice-President. This resulted in the two main competitors becoming President and Vice-President. They were often of very different opinions and unable to work well together. The 12th Amendment established separate ballots for President and for Vice-President. No person running for President would then end up as Vice-President to his opponent. This has resulted in the Presidential Ticket, in which each party nominates one person for President and one for Vice-President, and a team from a single party is usually elected to office.