Perhaps more important, though, is the simple fact that Southwest is an airline. The airline industry has historically been a poor performer for shareholders, for many good reasons. The emptier a flight is, the less revenue it generates. If one airline cuts fares on certain routes, others tend to follow suit, resulting in reduced profitability. The price of fuel frequently puts pressure on profitability.
Weather is a big factor.
A stormy season can decrease the number of people flying and the number of flights able to take off and land. Another consideration is the age of Southwest's fleet. The average age of its planes is currently just 8.2 years, which is fairly young for the industry. Southwest probably spends less on maintenance and repairs than its competitors. Still, this is a fleeting fleet advantage.