This is innate “caring” behavior.

Similarly, mammary glands are for providing milk/nutrition.

These get associated with primal needs and therefore the amygdala, and so is associated with strong emotion.

More than just "smooching," if DNA has anything to say about it...

For me it just seems like yet another culturally-defined signal of intimacy. Like showing ankles or chests or saying this or that. Seems to me trying to make everything some hard wired evolutionary thing is a dead end.