I disagree. But we all must write from the heart. Happy endings are nice when they work, and when they fit the story, but not all stories can end happily. Wouldn't we get bored?
I believe that sometimes an ending can be more powerful and meaningful if it provides a segue to something better or different. For example, think of a woman who comes to the realization that she must leave her husband, and finds herself packing a suitcase by moonlight with an owl calling outside. That ending would provide the suggestion of hope but certainly the end of a marriage and a woman escaping her husband in the night would not be considered a happy ending. Just something to think about. :-)