Yeah, like @TitsMcGhee said, it’s about intimacy, letting another cross your boundaries.
The mouth is extremely personal. We’re normally very protective of our faces; in some ways the face feels like the center of our being. And the mouth is like a gateway into our very interior. That’s heavy psychological juju. Allowing someone to cross that frontier is quite an expression of trust. And the fact that the other is yielding in the same vulnerable way leads to a feeling of melding and surrender. Psychologically, we harbor a wish to let down our defenses and feel the same intimacy that we felt as babies in our mother’s arms.
Kissing works as foreplay because it acts as a first test of the other’s commitment to trust before allowing the ultimate surrender.