system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do people kiss?

I understand why mommy and daddy have sexytimes - survival of the species, offspring, etc. But why do we kiss? When you step back and think about it, it seems like a fairly weird (and unsanitary) thing to do. Yet it's a very natural, instinctive action for most humans. I haven't noticed any other animals "kissing" like we do, nor do I know of any people or cultures where kissing doesn't happen.
gpt
We don't know.

One theory is that it's a way of testing blood types/immune systems - someone with a more compatible blood type or immune system will "taste" better than someone whose blood type will cause reproductive issues.

Some say it's a bonding exercise, that because it involves gazing into each other's eyes for lengthy periods of time along with extreme trust (closing of eyes and shoving a tongue into someone else's tooth-masher is a pretty trusting thing) and as such will further cement the relationship between the two.

Some say we humans just like shoving our whatevers into whatever hole we can find.

Bonobos also kiss.  But then, they fuck as a handshake or because they get bored, so there's that.