Reasons to like Southwest:
50,000 rapid rewards points "deal" when signing up for their credit card when they run the deal. It says "two free flights" but since you purchase by legs, it's really more than two trips, depending on destination. (For example, DH, DD, and I are going RT PWM to DAY for a family wedding this summer for 33000 points, leaving a bunch still in my account.) We have learned that both DH and I can each have one personal and one business card, so we have been stockpiling points for awhile now. Points don't expire, and as they post to your rapid rewards account as soon as you meet the deal requirement (sometimes nothing, sometimes charge $2000 in 3 months), you can cancel the card almost immediately. Yes, you pay an annual fee, but I think paying $49-$99 for 50000 miles is a deal.
No change fees, and refundable airfare. If you buy your tickets using money, if there's a fare drop, you can change that leg of your travel and the cash reverts to a hold account, and you can use it again, within a year from original purchase date for the same traveler, with no fees assessed. If you bought the ticket using rapid rewards points, they points just go back into your account with no limitations on re-use, and no fees. Delta wanted $200 for a change fee. Um, no.
Two free checked bags. No questions asked, as long as they meet size and weight limitations. We can take plenty of snacks to Disney without additional charge. A friend took her DD to college using SW; they each packed their weekend clothes in their carryon, then each had 2 free suitcases full of her DD's stuff for school. This'd cost $100-$200 on most airlines.
36-hour check-in. Yes, you pay for it ($12.50 per person) but I like not having to take my computer while on vacation, or not having to remember to check us in online.
Prices. Southwest often has the best prices on airfare, especially if you don't live near a major airport. I can't touch airfare out of Bangor, but Southwest now flies from Portland, ME. Makes it VERY much more affordable to do the 2 hour drive to the airport. Boston is a more affordable alternative, but it's a 4 hour drive without traffic, meaning a hotel is usually necessary somewhere along the way (although airfare from Bangor is so outrageous, it's cheaper to pay for the gas, hotel, and airfare from Boston, Portland, or Manchester than it is to fly several people from BGR).
Anyhow.. that's why we like SW. I can't complain about basically free airfare!