The question of how this came to be is not really in question. The moon is slowly moving away from the earth, as confirmed by laser pulses sent to retroreflectors left on the moon by Apollo astronauts. Eons ago, then, the moon must have been much closer, and the mass of the earth would have generated huge tides on the moon that gradually synchronized the moons rotation with its orbit.
That only answers how it happened, though, not why it happened. There only appear to be three legitimate answers to that question:
- A large moon near the earth is somehow responsible for earth being the kind of place where intelligent life could live. Venus, which is nearly earth's twin, has no moon, and it also lacks both a magnetic field and plate tectonics, both of which are needed for life as we know it to exist over the long term. Maybe the tidal force of the moon helped keep the earth stirred up enough for a magnetic field and plate tectonics. Maybe, but probably not; the absence of a magnetic field on Venus is probably due to its slow rotation, and its absence of plate tectonics is probably due to the fact that it has no water to help "lubricate" the rocks. (This really happens, but not the way it sounds. Rocks with water chemically bound in them behave differently than rocks with no water.)
Or maybe the moon in some way protected us from asteroid impacts by acting as a shield. Maybe, but probably not; the moon's gravity helps pull asteroids towards earth, and the center of gravity of the earth-moon system (to which asteroids would be attracted) is actually about a thousand miles beneath the surface of the earth.
- Maybe there is no particular reason for it. If there were no moon, we would be asking why there was no moon; if the moon rotated at a different speed, we would be asking why that happened. Even if all these things are equally possible, only one of them can happen.
- Maybe God arranged this for a reason, to teach us some kind of lesson. This is the idea I favor.
Seht ihr den Mond dort stehen?
Er ist nur halb zu sehen
und ist doch rund und schön.
So sind wohl manche Sachen,
die wir getrost belachen,
weil uns're Augen sie nicht seh'n.
Wir stolze Menschenkinder
sind eitel arme Sünder
und wissen gar nicht viel.
Wir spinnen Luftgespinste
und suchen viele Künste
und kommen weiter von dem Ziel.
These can be translated
Do you see the moon up there?
You can only see half of it,
all the same, it is round and beautiful.
The same goes for many things
that we laugh at without hesitation,
just because our eyes don't see them.
We proud children of man
are vain poor sinners
who do not know much at all.
We spin gossamers of air
and search for many skills
and further depart from our goal.
That, I think, is the best explanation we will get as to why the moon always shows us one face.