system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do dogs lick you when they're excited?
gpt
Here's a simple way to understand much of dog behavior:

Humans have evolved to walk on only two of their limbs, which frees up the other two for interacting with the world. We examine, explore, greet, embrace, and do just about everything with our hands and arms. So we move through the world on two limbs, and interact with the world with the other two.

Dogs need all four limbs for movement, which means they need something else for the behaviors we perform with our hands. So unlike us, they move through the world on all four limbs, and interact with the world using their mouth.

When we're excited to see someone, we usually shake their hand or give them a hug, performing some manner of physical interaction. Dogs can't do that, since their forelimbs are busy holding them up. So they lick you instead.