I recently came back from a trip where I flew on one of the major carriers (American) instead of a low-cost carrier such as Southwest. While I've read the troubles that plagued the airlines, I didn't realize how bad flying has become until I flew American. When it comes to low-cost carriers, I don't really think about the flight. It's like riding the bus or transit... it's just a mode of transportation. This is not a good thing to have your transportation be something you remember after your trip (imagine if your summer vacation is dominated by memories of that long painful drive). It could be that I don't expect much service on these flights and that is why it seems like the flight attendants are typically friendlier and happier, but on the major carrier the attitude is different. They want to distinguish themselves as higher then "low-cost carriers" at the same time that they reduce service and quality to below that of the low-cost carriers.
They try to be more formal, but because the service is subpar it just makes the flight uncomfortable. On my flight, I was asked by the flight attendant to step back 6 inches since I crossed the invisible line between coach and first class, and besides the fact that the flight only served a couple of drinks or sell you food the attendants somehow had to keep walking up-and-down the aisles throughout the flight so nobody can stand up.
In the end, flying the major carriers doesn't leave you with the feeling of having had a good flight. It doesn't even leave you feeling neutral and not noticing it. It is now decidedly un-enjoyable and unpleasant.