There is a case for happy endings. They're everywhere.
Sometimes, even I pick up one novel over another because I am looking for the predictable ending it will give me. Loads of genre fiction is like this. I might read a police procedural to catch the bad guys with the protagonist, or I might read a piece of high fantasy because I know that my warriors will defeat the forces of darkness. Most story arcs need a resolution. It is necessary to give the reader a chance to digest the subject matter. The peaceful closure at the end of these stories mentioned above makes me feel good because in general, this type of resolution works.
But, while happy endings as resolutions help create a smooth and easily-understandable novel, I think that an open-ended, uncomfortable finish can be more attractive to the reader in that it leaves us wanting something more. I'm searching for those right now. Something that makes me think.