Southwest shorted the market at the right time with regards to fuel. They're still buying oil at 40$ while everyone else is nearly double that, that is the way they've been so successful. Those contracts expire within the next 2 years, and then they'll have to play on the same field as everyone else.
Conversely, they have used the money for good things, like capital expeditures, they have nice airplanes, don't do the traditional spoke and hub, and pay their employees well.
Take a look at American Airlines, super-old fleet, union and employees and management all on different pages, use the spoke and hub methodology, are expensive to fly internationally.