why do couples sometimes show PDA when around a ‘single’ person?

I’m posting this on a throwaway because I don’t know how to ask this without sounding like a conceited douche. I (m 21) have never been in a relationship, but this observation happens frequently. I’ve noticed whenever I am near a couple in let’s say…. (waiting in line at a restaurant, serving people (I’m a waiter), sometimes at school) I notice either the male or female showing lots of pda, but doesn’t occur until I’m around. Again, I don’t wanna sound conceited , but I am not bad looking, dress well, and confident…. but I never “check out” anyone’s significant other as it’s genuine respect. Is this a sign of validation from both partners? Is this something couples purposely do?