Sentimentality is a quality of being overly, dramatically emotional — sad or loving or nostalgic. Your sentimentality on her eightieth birthday might make your down-to-earth grandmother roll her eyes.
When your emotions go overboard, becoming a little theatrical or false, that's sentimentality. A person's sentimentality might come in handy if a job as a greeting card writer opens up, or a role in a community theater's production of "Les Misérables." Sentimentality comes from the adjective sentimental, which is rooted in the Medieval Latin sentimentum, "feeling, affection or opinion."