Sometimes, it is even because they now want to wear the ring on a different finger.
So how exactly does resizing work, and more to the point, does it compromise the piece in any way? Not all diamond ring metals can be resized, but if you have a ring of yellow gold, white gold, rose gold or platinum, it will work. Metals such as tungsten, on the other hand, are not resizable, which means you should be all the more careful about the ring metal you pick. The alternative is to someday have to spend a few hundred dollars to get the metal.
For downsizing, metal is cut from the bottom, and what is left is joined through soldering. Upsizing can involve stretching, such as when the ring does not need to be made more than a little bit bigger, or cutting and adding metal at the bottom.