Sometimes we hear the "you're so skinny" comment being shoved as a compliment, when sometimes, it's anything but. We grow up aware that we must be careful with language that may be considered fat-shaming, however, we neglect to think about what we say regarding those who have thinner bodies.
Some people actually don't want to be thin.
Some thin people might be sick, terminally ill, depressed, going through treatment of some kind. Some thin people might generally have a difficult time gaining weight, although they really do try.
Some thin people might despise the thin bodies they are in, and may not want to be reminded of their size. I think it's extremely important to be sensitive to the varying issues concerning bodies. Since we do live in a world where thinness is the envy of bodies, we neglect to stop and think about how commenting on thin bodies may affect the person who actually lives in that thin body.