Happy doesn't mean always the life of the party - but rather looking on the bright side of things - dealing with the situation on hand, learning from it, and moving forward with a positive outlook. You want to look for people who "speak well of former employers, take personal responsibility for past career mishaps, and who will be okay whether or not they get offered your job". No whining or complaining allowed!
A big trait to hire for that often goes along with happiness is a can-do attitude. You want to find people who when given a task or a project, say 'yes, I can do that, and any parts that I don't know how to do right now, I will figure out'. Having employees who take the ball and run with it, rather than needing to be hand-led through every step, makes life so much easier for the manager, and also just gets more done! To find for can-do attitude, look for work projects or outside hobbies where they have taken initiative, ask applicants to complete screening tasks before hand, and get them doing real work during their interview so you can see how they react when on their toes. A good retail sales person can be helpful and work the floor in any store, even if they don't know the product line (although you also want to find someone who has a special interest in your product line and your company's line of business.)
Although remember that being happy and can-do doesn't mean always in agreement - you want to find hires that who will get along well with their boss and co-workers, but who will fill any voids in the organization's skill sets, values, or priorities, which sometimes leads to differences of opinion on what's important. A good leader can recognize where their weaknesses are (both personal and within their team), and hire people to step into those gaps, in order to fulfill on the organization's full mission.