It was said at the time that the JFK-Nixon TV debate changed politics forever. Nixon had 5 o’clock shadow and was sweating and Kennedy looked like a confident dashing stud.
I think it was the first televised presidential debate ever.
And yeah, Washington was known for not smiling due to the teeth thing. Look at any old photos, smiling was rare, people were told to freeze so as not to blur the image. Portraits of course are different but the big smile thing seems to be modern. Mona Lisa, anyone? Barely smiling and it was a big deal.
Part of it was that portraiture was considered a serious business. You wouldn’t sit for a painter with a s**t-eating grin, so you wouldn’t sit for a photo that way either.
Probably instamatics is what changed it. Once people were taking them on vacation everybody had to smile to show what a good time they were having, and it probably took off from there, all the way to Instagram.