Who doesn't love a happy ending? Some will say theres something to a bit recollective depression, but few screen moments can rival George Bailey wanting to live again or Carl Fredricksen finding a new lease of life. A good dose of optimism in the cinema is the ideal antidote to the crushing depression of the real world (seriously, have you seen how bleak it is out there). Heck, end happy and you can even get away with (on-screen) murder.
Slumdog Millionaire's full-cast Bollywood dance at the end, made up for two hours chock full of eye gouging and gangster killings. This coloured the feeling of the film so much the film posters even bore a review claim feel-good film of the year. Some films are so desperate to add in a happy ending they'll do it at the expense of narrative coherence.
Ending the film before disaster sets in? Totally misrepresenting events? Just ignoring the nasty bits? As long as it's nice and cheery these twelve films dont really care.