system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Speed causes time dilation. Mass causes time dilation. What is the link between speed and mass which means that they both cause the same phenomenon?

I hope the title makes sense.
gpt
To try to make this very simple, space and time are both components of a single thing called, very logically, spacetime. Spacetime is a coordinate system consisting of three spatial directions and one temporal direction. Time dilation is a result of the way things move through spacetime. Both speed and gravity are related to the way things move through spacetime, which is why they both cause time dilation.

For speed, again, in a very simplified way, you can conceive of everything moving at a combined “velocity” of c (“the speed of light”) through spacetime. Thus the faster you are traveling through space, the slower you are traveling through time, and vice versa.

Gravity, by contrast, is the warping of spacetime by mass/energy. Just as it warps the path that objects take when traveling through space, this warping also affects the path they travel through time. 

The mechanics are different, and the end result is also different (time dilation due to velocity in special relativity is reciprocal. Gravitational time dilation in general relativity is not), but the common thread is the interaction with spacetime.