I’m trying to remember this from a podcast I listened to a few years ago that covered this topic, so I might not have the details exactly right, but if I’m remembering correctly there is at least one evolutionary advantage in that there’s a virus (part of the herpes family of viruses, I think) that is asymptomatic and for the most part harmless, except when a woman contracts it for the first time while pregnant, in which case it can be pretty devastating to fetal development. But if the woman gets gradually exposed to the virus before becoming pregnant then her immune system learns to deal with it and it won’t harm the fetus. Of course that doesn’t explain why humans started kissing in the first place, but it could mean that humans who did engage in kissing may have had a significant breeding advantage.