I'm a total convert to SWA, and DH is even more adamant about it, thanks to a really ugly experience we had a couple of years ago w/ AA (our airport's other big carrier.) Other than the reasons Lewis and Gail already cited, here are some of the other reasons I prefer SWA.
Good prices.--Lots of direct flights to the places we want to go.--No ticket change fees.--Credit for the price difference if there is a cheaper fare available after you buy.--Reasonably priced one-way fares.--A FF program that ALWAYS lets you have the seat for the miles if there is a seat open on the plane.--Honesty about flight delays.--A willingness to work with any passenger who has a special situation and is polite about making the request.--Staff who don't routinely snarl at bewildered passengers.
The open seating policy is what allows SWA to consistently make a profit, and that, I truly believe, is what makes all of the above possible. If I could fly SWA everywhere I travel, I would.