(This question probably best fits in the DISCUSSION section of eNotes.)
In addition to the response of the above post, which I agree with entirely, I'd add that an imporant part of the fiction writer's task is to "create a world" that offers a shared space where the reader can actively engage the ideas of the work. This space is where meaning is made in literature. To create it, a writer must offer both the emotion (sentiment) and actual reasons, events, or circumstances that produce, justify, and deepen the emotion presented in the text.
Sentimentality offers very little opportunity for the creation of meaning beyond the specific emotions of nostalgia and the (overly) broad apprehension of the bittersweetness of life because "sentimentality" is by definition superficial:
Sentimentality is both a literary device used to induce a tender emotional response disproportionate to the situation at hand, and thus to substitute heightened and generally uncritical feeling for normal ethical and intellectual judgments...
Popular opinion suggests that the writer using sentimentality is "taking the easy way out":
"A sentimentalist", Oscar Wilde wrote Alfred Douglas, "is one who desires to have the luxury of an emotion without paying for it." Yeats wrote, "Rhetoric is fooling others. Sentimentality is fooling yourself."
Imagine a play where the actors burst into tears in the first act and never stopped crying as the play went on. There is emotion, yes, but there is no meaning behind it, no reality, no space for an audience to enter in and share the motivations for those tears.