According to federal statistics, airplanes are more crowded now than ever before. On average, most U.S flights operate above 80% of capacity; that empty middle seat is more likely than not to be filled. To compare, back in 1990 planes averaged 62% of capacity.

This means more people crowding the boarding gates, cramming in carry-on bags, and elevated stress levels from passengers and airline crew.

With such high aircraft loads, winter storms than cause flight cancellations make it much more difficult to rebook passengers on later flights, further irritating flyers. Read more: Here is a recent story from the Washington Post discussing the cause of higher flight loads, and other affects on travelers;
"Less free to move about the cabin."