The characters on ABC's new sitcom "Happy Endings" seem likable and funny. So why, the viewer may ask, does the show give them such a forced and convoluted back-story that it keeps getting in the way of both those qualities? One part of the answer, sadly, is obvious. TV has been flooded with so many "couples" sitcoms of late that each new one feels it needs to set itself apart by creating some memorably zany twist.