Just to throw in another opinion... I don't think PDA
always means a couple is showing off. If it's a new couple, especially if one or both haven't dated much, just being
able to rest your head on someone's shoulder or rub their knee might seem so wonderful and novel, because you've rarely had a person to do that with, that it's hard to stop whenever you're together. It's hard to be aware of yourself around others/in public when you're still reveling in the feeling of having someone to do this with. At least, I think this is true for some people. It doesn't make the outcome less annoying; but again, I like to think about the
why, as it helps me figure out what to do about it.
I remember, long ago, when my friend Amy had her first serious boyfriend. Fortunately he lived far away so we didn't have the PDA aspect, but she talked about him all. The. Time. I would say 95% of the things she said involved him somehow. Seriously. Anything anyone else said would remind her of something about this guy. "Wow, the weather is nice today." "Oh, Joe was just telling me how bad the weather was in his city..." "Pass the salt, please." "You know, Joe is on this low-salt diet..." And a lot of it was about how wonderful and perfect and amazing Joe was, and how
he thought
she was wonderful and perfect and amazing. "I talked to Joe for three hours last night and he said I was so smart and funny, and he couldn't wait to see what our children looked like, he hoped they had curly hair like me..." [Where's the "disgusted" face?] Nor did she change her behavior even after increasingly pointed comments from the rest of us. This lasted for the entire six months of their rel
ationship, until she flew out to see Joe special and he dumped her and started da
ting someone else two days later.

She did learn from it, though, and didn't act that way about any future boyfriends, even the guy she's now married to.