Short Answer
Prometheus steals fire to give it back to humans after Zeus took fire away from man. According to the greek mythology Zeus decreed that for every sacrifice that man made to the gods, the sacrifice should be split into two portions one for man and one for god.
Here Prometheus decided to trick Zeus he split the sacrifice into two piles one having the good meat covered with animal hide and the other with bones covered with juicy fat.
Zeus fell for the trick and picked to pile with the bones and fat, In his anger over being tricked he took fire away from man.
Out of love for humans Prometheus decided to raid the workshop of Hephaistos and Athena and give the fire back to mankind.
Overview
Prometheus is one of the titans in greek mythology, also known as the god of fire. Prometheus was the son of the Titan Lapetus and the Oceanid Clymene.
A titan himself he along with his brother Epimetheus decided to side with Zeus in the great war that occurred between the Olympian gods and the Titans.
For this very alliance the Olympian gods spared their imprisonment in Tatarus and further gave them the task to create and populate the earth with diverse animals.
While Epimetheus was busy creating animals, Prometheus from his own hands shaped man out of clay and Athena breathed life into his clay figure.
Epimetheus was made responsible for giving various qualities to animals, but by the time he got to man there was nothing special left. So Prometheus took it upon himself and decided to make man unique among all living things.
To make man stand out over all animals Prometheus decided to gift them intelligence and the ability to use fire.
With fire man started to develop rapidly, now they were able to fend off animals, make tools and weapons, soon man began to dominate nature.
Reason Why Prometheus Stole Fire
Watching this new creature develop rapidly made Zeus uncomfortable as he didn’t want man to be in equal standings with the gods. He believed that if humans would be any stronger they would challenge his absolute control over humanity.
To assert his dominance over humanity Zeus decreed that they honor the gods by offering sacrifices. A meeting took place at Mecone, to settle the division of sacrifice between gods and men. Here Prometheus tricked Zeus for mankind’s benefit.
The story goes like this, Prometheus slew a large ox, and divided it into two piles. One pile had all the meat and was covered with the ox’s hide, the other bigger pile had all the bones and was covered with shining fat.
The cunning Prometheus knew that Zeus’s ambition would make him choose the bigger pile leaving the best pile for man.
Upon knowing that he had been deceived Zeus became furious and decided to hide fire from mankind, leaving them cold and shivering at night, Soon man started to regress.
Out of pity and his love for mankind Prometheus decided to raid the workshop of Hephaistos and Athena on Mt. Olympus and steal fire. He brought the fire back to man by hiding it in a hollow fennel-stalk.
The Punishment
With the access to fire once again mankind started thriving once more. When the glowing flames at night attracted Zeus’s attention, He became outraged and decided that Prometheus could not be left unpunished and had to make an example out of him for those who defy Zeus’s decision.
Zeus imprisoned Prometheus on Caucasus mountains where a giant bird would devour his liver every day. As Prometheus was an immortal Titan his liver and skin would regenerate overnight just to be eaten by the bird the next day.
This left him to endure excruciating pain again and again and again day after day.