In a prime example of Chrony-capitialism, Obama is opposing the XL pipeline as payback to his buddy Warren Buffet.
Mr. Buffet's railways currently carry a majority of the oil transported by rail from Canada. If the XL pipeline was built, Mr Buffet would lose a lot of money. If Mr. Buffet loses money, Barack Obama loses a massive political finance contributor.
Just one of the many examples of how Obama is willing to let the people suffer for the "Greater Good".
Obama didn't veto XL Pipeline bill because of anything to do with Warren Buffet, he did it for political reasons. Because Congress is Republican-controlled, he had to 'take one for the team' and veto it. It would've been very bad for the Democrats if he hadn't vetoed it.
If anything, this situation is a clear example of the dysfunctional state of the American political system. The veto power was a bad idea from the framers of the Constitution! They clearly didn't think about the system they were creating. It was based on a romantic ideal of America being the new Roman Republic, with the President as Consul (who had veto power. In fact, James Madison refers to the President as a 'Chief Magistrate' in the Federalist Papers - a term used in Roman times). Gosh! This is a prime example of where big ideas can get you into trouble! They should've stayed with the British parliamentary system. They'd have been a lot of better off than what they are now!