Why do people think X when they used to think Y?

Elvis Presley...

In his time, Elvis's dancing was thought by most people to be lascivious. I was reminded of this when Fess Parker died (Daniel Boone actor). Parker said the Daniel Boone show was more popular than Elvis for most people because it was clean. We no longer think of Elvis with shock, but a sort of quaint 50's nostalgia.

Michael Jackson? When he became popular, I was in junior high school. The way he thrusted made even me uncomfortable. But today he is the King of Pop and a legend, a marvelous original dancer, almost heroic, and definitely beloved.

Marriage is less and less popular, but used to be an almost universal goal of people. Witness from song lyrics: 

Beach Boys Wouldn't It Be Nice

Wouldn't it be nice if we were older

Then we wouldn't have to wait so long

...we could be married

then we'd be happy

The Hollies Bus Stop:

Some day my name and hers are going to be the same

...Nice to think that that umbrella Led me to a vow

Manfred Mann Do Wah Diddy.

She looked good (looked good), Looked fine (looked fine)

Looked good looked fine, Wedding bells are gonna chime

Then:  dressing up to fly on a plane. My first flight in 1977 was like dressing for church.  Now:  no dress code. Vacationers wear bikini's on a plane

Would the advertising slogan "An Army of One" been successful for military recruitment slogan in 1950?

"Making love" used to mean hugging, kissing and or sweet talking. (eg, in "Its a Wonderful Life", Mary shouts up to her mother upstairs about George Bailey: "he's making violent love to me, mother!" She meant wild kissing, nothing else.)

School class photos before 1980: Then: colors, prints, stripes, checks. Clothes were just clothes. And equal ratio of skirts and slacks, no logos. Now: hard to find a shirt without a logo or slogan on it.

Star Wars was mind-blowing in 1977. Now it takes Avatar to wow us.

Underwear: I grew up wearing monochrome briefs.
Why do boxers now come in patterns and colors? Implication: they are meant for viewing.

What is going on when some girl's shorts have letters right across the butt, such as "CHEER". I saw one today that said "bebe".

Christians believed in hell.

You interacted with an gas station attendant when buying gas.

Parents felt safe letting their children walk to school or ride a bike without a helmet.

Smoking used to be acceptable and drinking was frowned on. More or less reversed today.

Children's movies and cartoons never mentioned farting, vomiting, belching or underwear.

Homosexuality was listed with the AMA as a mental illness.

'Hate' used to be a word that was too strong for everyday use.

Boys aspired to military service and joined the Boy Scouts when they were too young to join the army.

Keys used to be left in cars, houses left unlocked.

Drawing, music, and needlecraft were normal activities for girls.

Poetry had rhyme and meter.

Books were made of paper.