Why do we always see the same side of the moon?

It is tidal locking that causes the synchronous rotation which causes the Moon to present "just one side" to the Earth all the time. The Moon rotates (spins) but very slowly, making one turn on its axis in the same time it takes to revolve (orbit) around the Earth. So it maintains a "constant face" in our direction. We actually see slightly more than half of its surface as it turns.

It is believed that the Moon originally had a faster rotation (spin), but the effect of the Earth's gravity was to reduce the spin.

(The dwarf planet Pluto and its moon Charon are believed to be "dually locked", so that each shows the "same side" to the other as Charon orbits.)