I still listen to them daily, and I still sing along with every single song. Honestly, you can't help not sing to those ballads. The songs are short and to the point, usually with the same verse being repeated over and over and over, so it isn't that difficult to know every song by heart.
I had a friend in the car one day as the cassette played and she made a comment that immediately rang truth: "those songs are made for simpler times and a simpler world".
Well... it isn't entirely true seeing as the Vietnam war was underway, Japan was still going through its post A-Bomb reconstruction and Egypt was still in a state of War with you-know-who and had just witnessed its greatest defeat at their hand. "A simpler world" isn't really the right description, but we get the gist of the comment: musical tastes were simpler and music itself was more to the point.
Take the song Michelle for instance: who on our planet today could ever get away with a song in which the chorus says "I love you, I love you, I love you" in the cheesiest way possible? Some might... but I doubt they'd win a Grammy for it and I guess it would hardly ever be the 42nd most played song in the world.
Rumor has it was John Lennon who suggested this "I love you" bridge to McCartney (who was the mastermind behind the song that was intended originally as a slight mockery of the French Rive Gauche culture that was taking England by storm).