His/her tears are shallow.
Some people hardly ever cry at all, even when they are profoundly sad or moved. They have the ability to fight back the tears and keep their eyes dry.
Others can cry in an instant, for the simplest of reasons. It doesn't take them much to shed a tear. It's as if the tear was already on hand, just waiting for the slightest reason to come out.
These people are said to be nátata lago'-ña. Their tears are shallow. Why?
Their tears are right at the surface of their eyes. Any closer to the surface and their eyes would be spilling out tears all day long.
In order for these people to cry, they don't need to reach into the deep recesses of their storage of tears. Their tears are right there, on the surface, shallow, ready to shed at any moment.