Cockatiels are intelligent, sociable birds, and when their needs aren't met they may act strangely. One example of strange behavior is pacing: if your bird paces back and forth in his cage, he may be distressed, bored or even sick. Monitoring his behavior and taking stock of his needs may help you understand the reason for his abnormal pacing.
Domestic cockatiels become close with their owners—very close. If your bird only starts pacing whenever he sees you, he may be acting out of sheer joy, as he's excited that you're around. He may pace back and forth on his perch or even scale the walls of his cage, channeling his excitement at seeing you into his behavior and trying to get your attention.
A cockatiel doesn't like to be cooped up all the time. He needs to come out of his cage at least once a day—not just to be with you and socialize, but to get a break from the monotony of being locked up. This doesn't mean that cockatiels don't appreciate their cages; a cage can be a safe, secure and comfortable space for your bird. However, he needs to get out and stretch his wings every day, or he's going to get anxious. That anxiety can translate into behavior like pacing.
Your bird has a sharp mind, and he needs a way to exercise it. Just as you wouldn't want to be stuck in a room all day with nothing to do, your cockatiel doesn't want to get bored in his cage—if he does, he may resort to monotonous activities like pacing to entertain himself. Make sure that he has toys in his cage, and switch them out every few days so that he doesn't get bored. A little mental stimulation can make your bird considerably more comfortable in his cage.
Abnormal behavior like pacing and demonstrating unusually high energy levels can indicate a health problem in your cockatiel. If you notice him pacing, monitor him for other common symptoms of illness, like a loss of appetite, excessive feather plucking or unusual thirst. Report any symptoms you observe to your veterinarian, who may need to perform an examination to make sure the pacing isn't caused by a physiological problem.