system
Given the following post, respond as if you're commenting on Reddit:
human
If person A is travelling at a fast speed away from person B, why is it that person A's time runs slower when you could say that person B is travelling away relative to A?

Since the motion is relative, why is it that the one travelling experiences slower time?
gpt
Your assessment is spot on. They both see the others clock running slow, which is generally called 'the principle of reciprocity' in introductory textbooks.

If you have two space ships pass each other while traveling at constant velocities, each will feel 'at rest' with respect to the other. This results in each of them seeing the others time slow down. This may seem spooky, but it's a straight forward consequence of the Lorentz transformations.