My opinion, based on everyday experience with a full-service U.S. major:
Unrealistic pax expectations are almost totally attributable to the misleading marketing practices of the full-service majors who like to make their customers think that they will "get more for the same price," when in reality, they will almost invariably get about the same service for the same price or often about the same service for a higher (sometimes much higher) price or even poorer service for a higher price.
Also, the full-service U.S. majors seem to be largely duped into supposing that air travel is a one-way street and the airlines, desperate for business, need to cave in to the totally unrealistic, sometime outrageous demands of their pax and, thus, chase good money with bad, all in the interest of assuming that they will keep "desperately needed" obnoxious, unprofitable pax flying with their airline instead of another.
By contrast, low-cost airlines tend to have a more realistic two-way street perspective: "yes, our airline needs customers, but pax also need the service our airline provides." Therefore, their approach is something like "we won't play games with you - neither will we allow you, our customer, to play games with us."
That, IMO, is why low-cost airlines, especially Southwest, are consistently rated at or near the top in customer satisfaction ratings - in spite of all the frills they don't offer. Seems like the latter approach also tends to cause pax to be much more likely to take ownership (i.e., responsibility) in doing their part to ensure a satisfactory travel experience.
One final observation concerning pax expectations at the full-service major by whom I am employed: passenger expectations are almost invariably in inverse proportion to the fare paid; those who pay the very least have the most ludicrous expectations while those who pay the most expect little if anything more than a safe, on-time flight from A to B.