When people kiss someone different from them, that person to some extent represents more than just themselves. They represent all Russians, all lawyers, all only children, etc. By kissing one (or a few) people feel like they have made contact with a community through a person. And not just any contact, but a sumblime mutual appreciation. By kissing a dancer, for example, I can say "Dancers like me! Dancers kiss me! And in this moment, I am making out with Dance!"
The largest such group is the group of strangers. By kissing a stranger, I am kissing all of the humanity I don't know. This may have a special significance living in a city because I am surrounded by hundreds of trillions of strangers. We barely acknowledge each other and we don't know each other's name, but by kissing a stranger I get to feel that, though we pass quietly on the street and in the subway, we like each other. And that makes the city's strangers into kind of a mysterious wonderful place to be.
The Panel: Buffy, Gun Street Girl, The Gentleman, Chicken