Chick-Fil-A employees are so happy for a few reasons, such as individual personality, company culture, and job requirements. When hiring, Chick-fil-a looks for individuals who have outstanding customer service skills and are pleasant and happy individuals.
After hiring, people are often reminded to continue to be happy on the job. So, these naturally happy people are also often pushed to appear even happier while on the clock.
Now, as far as working in a fast food environment, it is much easier to be happy at Chick-Fil-A as the company has set up the business to have the least amount of "issues" generally present in fast food establishments. It is not open late, which can lead to a lot of problems. They also are paid well and are given great perks.