Why did Zeus not want Prometheus to give man the gift of fire?
Prometheus steals fire from the gods with the express intention of giving it to humanity so that it can create civilization. Fire was also considered important at the time as it allowed humans to create metalwork, an essential component of ancient weaponry. Zeus is enraged by Prometheus's actions and punishes him by having him chained to a rock where his liver will be pecked out by an eagle every single day. The liver will grow back, only to be pecked by the eagle once more the following day.
Zeus is angry at Prometheus for two main reasons. First of all, Prometheus was originally a Titan, one of the antagonists who engaged in a mighty ten-year struggle with the Olympians—led by Zeus—for control of the heavens. Prometheus switched sides and ended up with the victors, but his reputation for being a trickster is clearly well-deserved and Zeus is furious at being fooled by the Titan turncoat.
More importantly, Zeus doesn't want humans to have fire, because it will make them stronger. He wants to maintain absolute control over humanity, for mortals to be under his complete command. Although mortals will still remain in a position of subordination to the gods, even with the gift of fire, Prometheus's bold, brazen defiance of Zeus, brings human beings just that little bit closer to the level of the gods, and the father of the gods cannot tolerate this.
Zeus' primary fear of giving man fire was that he didn't want man to be in equal standings with the gods. Zeus believed that fire, and its mystery, was one of the elements that allowed humans to stand in reverence of the gods. His belief was that as long as human remained in "darkness" (fire= light), they would be submissive to the gods' will. He believed that giving man fire would cause the gods to have lost some of their "lustre" in the eyes of man. In some respects, he was right, for once Prometheus gave fire to man, he also told them how to trick Zeus using fire and animal sacrifice. For his part, Zeus did not take too kindly to be ridiculed and punished both man and Prometheus in a very swift and brutal manner.
When Prometheus created mankind, he only cared about the men. Zeus was really upset, and feared that mankind would become too strong, and even warned him not to give mankind the power of fire. When Prometheus refused to listen, Zeus was enraged, and stole fire from mankind. He thought that men did not deserve to live in light, but should live in eternal darkness.