Whether creating a "happy" ending or one in which the wronged protagonist exacts revenge upon the antagonist, the instigators of these alternate endings tend to ruin the spirit of the original work. In Henry James's novel Washington Square (1880), Catherine is wronged by her supposed fiancé, Morris Townsend. James ends the novel with a dignified Catherine simply saying to Morris, "Please don't come again" (164). The film, The Heiress, has Catherine (Olivia de Havilland) trick Morris (Montgomery Clift) into thinking that she will marry him, locking the door, retreating up a long case of stairs, and leaving Morris desperately pounding on the door. And the audience raves because Catherine gets her revenge. Similarly, A Streetcar Named Desire is changed. Tennessee Williams' intended ending is altered ever-so-slightly so that, instead of Stella (Kim Hunter) remaining with her chauvinistic, rapist husband, she instead leaves him. Again, audiences rave. In this case, the film makers felt that it would be wrong to end the film any other way.
In the film My Fair Lady, the ending is also changed so as to yield the quintessential "happy" ending. Author George Bernard Shaw insisted that his play Pygmalion (1913) was not a love story; nevertheless, when it was adapted into a musical play in 1956 by Alan J. Lerner and Frederick Loewe, which was later filmed in 1964, the overall focus was made to reflect a romance between the two main characters, Eliza Doolittle (Audrey Hepburn) and Henry Higgins (Rex Harrison. Shaw specifically made the focus in the original play so that Eliza did not return to Higgins, yet the film embodied the opposite.
These alterations reflect what the average reader would deem a "proper" conclusion. In fact, I confess that, while reading the original works, I too felt that they should have ended differently--in a perfect world! But the authors' original endings reflect life's reality. Would Stella actually leave Stanley?--probably not. Would Eliza be better off with Freddy?--probably so. Yet, regardless of those who may find these changes annoying, well, it is the movies.