system
Given the following post, respond as if you're commenting on Reddit:
human
Is it a coincidence that the moon is tidal locked or did something cause it to be that way?
gpt
No coincidence. 

As long as the Moon, no *any* moon, is not tidal-locked, it undergoes tidal forces from its planet, i.e. so its mass get shuffled a bit around (picture that as the moon having a slighly oval shape, i.e. not spherical but elongated toward its planet, and this elongation, following the planet, is always along a different axis). Naturally, this movement of matter (most noticeable with water, which is quick to reshape, but present in varying amounts with any substance) costs enegy. Who is "paying" for it? Answer: the kinetic rotation energy of the moon. In other words, tidal forces dissipate kinetic rotation energy, ultimately turning it into heat (through friction). 

So, tidal forces on the moon cause the moon to slow down its rotation around itself. If this goes on for long enough, and for our Moon it did, the moon slows to the point that no tidal force exists anymore. That's when elongation direction is kept always the same with respect to the moon. That's when the rotation of the Moon around itself is the same period as the revolution of the Moon around Earth; the Moon got "tidally locked" to the Earth.

BTW the same is happening here on Earth: tidal forces caused by the Moon "consume" Earth rotation energy, making it slow down a bit.  Incidentally, this is why we can actually extract energy, e.g. electricity, from tidal forces. Just has it has heppened for the Moon, Earth revolution period (currently 24h) is getting longer and longer; it was ~22h hours when dinosaurs were around. When it will be ~28 current-days long, Earth will be tidally locked too. Which is never, because Earth won't be around long enough for that to happen.

Whether or not that happened for two given bodies depens on how much rotation energy they stored (which depends on mass and rotation speed), how much energy the tidal forces dissipate (which depends on composition, e.g. water dissipates a lot more than solid rock), and how much time they got to get tidally locked.

(Ok I've cut a few corners here. The most important one is that even if the rotation speed of the Earth decreases, the angular momentum of the system is kept constant -- which means that as the rotation gets slower, the two bodies get further apart from each other. The moon is slowly getting smaller in our sky.)