In my experience, it's not so much about flying being "fun". The shine always wears off at some point. But there are pros and cons to it like every other job. A big pro to flying 121 is that your job duties are crystal clear relative to office work. For the most part, your duties are not as open to influence by the next big "cost savings measure"...you're still in charge of flying the plane from A to B. You're not going to be running the fuel truck and grabbing the coffee for passengers because some bean-counter got the not-so-novel idea to reduce staffing once again. To some people, the repetition in flying is tedious and boring. It all depends on what you value.