My cousin wrote a post called 'Care for happy endings?' in which she wrote about how she feels cheated by movies which don't end on a happy note.  While I love happy endings as much as any other person, I'm of the opinion that the ending of a movie has to be one that does justice to the story, one that adds substance to the movie and not one that takes something away from the experience.  Any attempts to goad the story to a happy conclusion when the soul of the movie innately steers it elsewhere, just to cater to people's expectations, is a bad idea according to me.  That said, many directors go ahead and do exactly that: beat the story into a conclusion which can meet the approval of the masses.  Rare are directors who do their own thing and risk angering movie-goers in trying to stay true to the soul of the story and of them,  fewer are those who have managed to get the people to see their view point.  While some movies naturally gravitate towards a happy ending, others simply don't.  Shrek and Shrek 2 are examples for the former: in the first part, Princess Fiona turns into an ogress and to her surprise discovers that Shrek is more than overjoyed to take her into his life, just the way she is! In part two,  though Shrek is able to transform himself into a handsome man,  Fiona doesn't choose to transform herself as she loves the shy and bumbling ogre to the handsome stranger she sees.  Both were lovely movies with perfectly happy endings.  However, there are stories which gravitate towards sadder conclusions and they ought not to be tampered with. 

For movies with sad endings which simply would have been completely disastrous, if the ending had been different, let me call upon two examples. The first is Anand, an absolutely wonderful Hindi movie starring the late Rajesh Khanna as Anand Sehgal, a terminally ill cancer patient with a never-say-die personality.  The movie ends with Anand dying and Bhaskar Banerjee, Anand's doctor,  is shown presenting Anand's story to a highly appreciative audience. Now here's a what if:
How would it have been, if Dr. Banerjee had come up with a miracle cure that ensured Anand's full recovery and ended up getting a Nobel prize in front of a thunderously applauding audience? One word: Terrible.  Why? Here's why:
1) The audience was being led towards the impending death of Anand, right from the beginning.  His quirks and eccentricities, which otherwise would have had him labelled a lunatic were accepted as examples of his 'sunny disposition', only because the audience knew he was dying.  If he recovered, the audience would only feel cheated!
2) The alternate ending would have completely taken the focus of the movie away from Anand and directed it towards the doctor who cured a terminally ill cancer patient, and that was not the intended effect at all!
3) The movie would have been panned by critics for being insensitive to millions of patients suffering from cancer, as it would give rise to false hopes.

Second example? V for Vendetta. In the end of the movie, V is fired upon by multiple gunmen, cronies of the evil Mr. Creedy.  After V finishes off all his henchmen, an exasperated Creedy asks V "Why don't you die??!"  "Beneath this mask there is more than flesh. Beneath this mask there is an idea, Mr. Creedy, and ideas are bulletproof." is V's reply. V, after killing Creedy, succumbs to his injuries, in Evey Hammond's arms.  Evey goes on to complete V's planned demolition of the parliament house and people, inspired by the event, decide to rise up against all tyranny.  Now here's a what if:
What if V had emerged unscathed and gone on to remove his mask and kiss Evey Hammond? Would we have had a wonderful conclusion? No! Why? Here's why:
1) The entire movie was about ideas and how ideas are more important than individuals.  The ideas of Guy Fawkes and V lived on, long after they ceased to exist.  If V survived, the conclusion would be damned!
2) The movie was not about the romance between V and Evey, though it did play a crucial role.
3) Right through the movie, V doesn't take off his mask as his face was supposedly horribly burned.  If it were to be a happy ending, the focus would go to whether V would show his horribly burned face to Evey to kiss her, or kiss her with his mask on, his cause and idealogy be damned!

Stories should be allowed to flow towards their natural conclusions and not get doctored, to suit popular tastes. I rest my case.