It is similar to hugging or any close physical contact – when an animal lets another animal close enough to them that they *could* bite/attack/harm them, it shows trust. The closer it is, the more trust you display.
When you think about it, there aren’t that many ways we can physically display trust. Making yourself vulnerable is the easiest way to do it
I often think about how almost everything we do and are is really weird when you give it thought. Everything seems pretty well put together, pretty convenient, well constructed, but also just not perfect enough to be unbelievable. Just look at sports, most sports are pretty silly when you think about it.
Physical touch, in general, can release hormones that make us feel calm or happy. Despite this, however, it’s not universal for people, either by culture or as individuals, to see kissing as an intimate, or even typical, act. There are actually cultures that don’t engage in intimate kissing, some aspect of it really is just social and the fact that we normalized it as being an intimate act in the West at some point
Edit: should say that I’m operating off of some pretty foggy memories here, so take it with a grain of salt
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