Justification for this as a PhilosophySE questions: there are two SEP articles concerning this topic:
Carroll seems to concur with my intuition, here, although I can read only the first page of this text, so I don't know that the apparent concord endures to the end of the matter. My intuition otherwise comes from a sense of count nouns as involving literal countability (in the set-theoretic sense of "countable"), with mass nouns mapping onto uncountably multipliable bits of matter/similar. However, if I push this too literally, I would seem to commit myself to the presence of an uncountably infinite amount of e.g. water in any proper mass of water, which (indeterminably-many virtual particles constitutive of Dirac water aside) seems false.
But could it instead be said that water is a mass because it could be expanded to an uncountable multiplicity of presence, whereas things denoted by count nouns cannot be so multiplied? For example, an individual dog cannot be multiplied so (I suppose), though its dogness might admit of this?