Why did lord Shiva drink the poison

Lord Siva is addressed here as blue throated. Lord Siva drank an ocean of poison and kept it in his throat, not swallowing it or allowing it to go down to his stomach, and thus his throat became blue. Since then he has been known as nilakantha, or blue throated. The reason that Lord Siva drank an ocean of poison was for others' benefit. When the ocean was churned by the demigods and the demons, the churning at first produced poison, so because the poisonous ocean might have affected others who were not so advanced, Lord Siva drank all the ocean water.
The summary of the Seventh Chapter is as follows. As described in this chapter, the Supreme Personality of Godhead, appearing in His incarnation as a tortoise, dove deep into the ocean to carry Mandara Mountain on His back. At first the churning of the ocean produced kalakuta poison. Everyone feared this poison, but Lord Siva satisfied them by drinking it.
With the understanding that when the nectar was generated from the churning they would share it equally, the demigods and the demons brought Vasuki to be used as the rope for the churning rod. By the expert arrangement of the Supreme Personality of Godhead, the demons held the snake near the mouth, whereas the demigods held the tail of the great snake. Then, with great endeavor, they began pulling the snake in both directions. Because the churning rod, Mandara Mountain, was very heavy and was not held by any support in the water, it sank into the ocean, and in this way the prowess of both the demons and the demigods was vanquished. The Supreme Personality of Godhead then appeared in the form of a tortoise and supported Mandara Mountain on His back. Then the churning resumed with great force. As a result of the churning, a huge amount of poison was produced. The prajapatis, seeing no one else to save them, approached Lord Siva and offered him prayers full of truth. Lord Siva is called Asutosa because he is very pleased if one is a devotee. Therefore he easily agreed to drink all the poison generated by the churning. The goddess Durga, Bhavani, the wife of Lord Siva, was not at all disturbed when Lord Siva agreed to drink the poison, for she knew Lord Siva's prowess. Indeed, she expressed her pleasure at this agreement. Then Lord Siva gathered the devastating poison, which was everywhere. He took it in his hand and drank it. After he drank the poison, his neck became bluish. A small quantity of the poison dropped from his hands to the ground, and it is because of this poison that there are poisonous snakes, scorpions, toxic plants and other poisonous things in this world.
 Lord Siva said: My dear Bhavani, just see how all these living entities have been placed in danger because of the poison produced from the churning of the ocean of milk.
 It is my duty to give protection and safety to all living entities struggling for existence. Certainly it is the duty of the master to protect his suffering dependents.
People in general, being bewildered by the illusory energy of the Supreme Personality of Godhead, are always engaged in animosity toward one another. But devotees, even at the risk of their own temporary lives, try to save them.
 My dear gentle wife Bhavani, when one performs benevolent activities for others, the Supreme Personality of Godhead, Hari, is very pleased. And when the Lord is pleased, I am also pleased, along with all other living creatures. Therefore, let me drink this poison, for all the living entities may thus become happy because of me.
 It is said that great personalities almost always accept voluntary suffering because of the suffering of people in general. This is considered the highest method of worshiping the Supreme Personality of Godhead, who is present in everyone's heart.
 
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