From Jacques Barzun’s From Dawn to Decadence: 500 years of Western Cultural Life, 1500 to the Present, pages 410-11:
But what is sentimentality? If one asks somebody who ought to know, one is told: an excess of emotion; or again, misplaced emotion. Both answers miss the point. Who can judge when emotion is too much? People vary not only in the power to feel and express feeling, but also in imagination, so that a stolid nature will deem it excessive as soon as love or grief is expressed vividly and strongly. Shakespeare is full of “exaggerated” emotion, but never sentimental. The same remark applies to the other answer. When is feeling misplaced? at the sufferings of the tragic hero? at the death of a pet? at the destruction of a masterpiece? One may argue that any emotion out of the common should be restrained in public, but that is another answer, one of social manners that has nothing to do with a feeling’s fitness to its occasion. The diagnostic test must be found somewhere else.
Sentimentality is feeling that shuts out action, real or potential. It is self-centered and a species of make-believe. William James gives the example of the woman who sheds tears at the heroine’s plight on the stage while her coachman is freezing outside the theater. So far is the sentimentalist from being one whose emotions exceed the legal limit that he may be charged with deficient energy in what he feels; it does not propel him. That is why he feels pleasure in grief and when he is in love never proposes. Sterne accurately entitles his story A Sentimental Journey: the tears he shed over the death of the donkey and his preoccupation with the girl at the inn caused him no upset nerves, no faster pulse or quickened breath. He reveled in irresponsible grief and love. This condition explains why the sentimentalist and the cynic are two sides of one nature. In such matters the arts are transparent and the connoisseur can easily tell imitation feeling from the real thing.
classical life 
Makes sense to me. How about this very different take from John Adams’ program notes for the premiere of “Naive & Sentimental Music” . . .
“Naive” and “sentimental”: I use these two terms knowing they may at first be misunderstood I mean them not as we commonly interpret them but rather in the sense that Schiller used them in his essay “Über Naive und Sentimentalische Dichtung” (“On Naive and Sentimental Poetry”), a once-influential essay from 1795 which has by now been all but forgotten. Schiller saw essentially two types of creative personalities: “those who are not conscious of any rift between themselves and their milieu, or within themselves; and those who are so concious.” (I quote from Isaiah Berlin, who so succinctly summarizes Schiller’s point of view.) The “unconscious” artists are the naive ones. For them art is a natural form of expression, uncompromised by self-analysis or worry over its place in the historical continuum. “They see what they see directly, and seek to articulate it for its own sake, not for any ulterior purpose, however sublime.” Schiller cites Homer, Shakespeare, Cervantes and his own contemporary, Goethe, as examples of the naive. Opposed to this is the sentimental poet whose art “comes about when man enters the stage of culture where the primordial, sensuous unity is gone…The harmony between sense and thinking , which in the earlier (naive) state was real, now exists only as an ideal. It is not in a man, as a fact of life, but outside him, as an ideal to be realized.” The sentimental voice arises when the unity has been broken, and the poet (or composer or painter, etc.) seeks to restore it or, going to the opposite extreme, parodies or satirizes it. In Isaiah Berlin’s words, the sentimental artist “looks for the vanished, harmonious world which some call nature, and builds it from his imagination, and his poetry is his attempt to return to it, to an imagined childhood, and he conveys his sense of the chasm which divides the day-to-day world which is no longer his home from the lost paradise which is conceived only ideally, only in reflection.” For Schiller the poet “is either himself nature (and thereby naïve), or else he seeks nature (and is thereby sentimental).
Yes, I remember that. That, of course, is a very different use of the term “sentimental.” Barzun discusses Schiller’s essay, btw, in From Dawn to Decadence, and he suggests a different term for Schiller’s “sentimental.” I wish I could remember what it is … but I don’t have the book at hand at the moment.
Jacques Barzun above states; “Sentimentality is feeling that shuts out action, real or potential.” I agree with this but would like to offer another way of stating it that may allow for a more accessible definition for recognizing it. In sentimentality there is always one (or more) relevant value that is considered sacrosanct or unquestionable. It could be mother love, apple pie, patriotism, the American Way, God’s love, individualism, socialism, positivism or any “ism” or religion or school of philosophy pet value of the writer.
I would suggest that sentimentality is the most dangerous drug in the U.S. Worse than heroin, cocaine and the other chemical ones. It goes well with these and other drugs however – think of how well it goes with alcohol for example. Very possibly the worst purveyor of this drug was Walt Disney. Compare his version of “Beauty and the Beast” with Jean Cocteau’s version. There is a profound difference between “sentiment” and “sentimentality”. How many times does one hear a movie praised because it is a “feel good” movie? Feel good films or books do not change the world. Subliminally it is easy to slip into the comfort that someone else will take care of the problem but see what Rachel Carson’s “Silent Spring” started. That book came out of a powerful emotional and intellectual reaction and discovery. There was no sugar coating. It was not a feel good experience.
In this country the belief in the righteousness of the U.S. is never questioned by the major parties. None of them considers that we are becoming, possibly if not probably have already, become the most evil influence on the planet. In a so called “democracy” or “democratic republic” one never hears Communism, Socialism or even the nature of a true democracy discussed openly in the media of a supposed free society or why all the bankers are back in business and doing wonderfully while the rest of the country does poorly. What objective discussion was there of the concept that some companies or banks were “too large to fail”?
We go to bed, deep down inside we swallow that with all its faults this is still the best country without really questioning it. That is one fruit of a continual diet of not seriously questioning the basic value of the American Way.
To cleanse ones self of the sentimentality that runs through American culture read the Greek plays of Aeschylus, Sophocles and Euripides and consider after you experience them that the common citizens of Athens filled the theaters.
Or that the tragedies of Shakespeare were experienced by the groundlings – watching them while standing.