Why snake is seen around the neck of God Shiva ?

 According to puranas , demons and devas both agreed to churn the milky ocean

 in order to obtain the amrit .

They used serpent king Vasuki as the rope for churning . But before they get amrit

 , there was the creation of poison called halahal.

God Shiva consumed this poison . As the poison was being coming out because

 of churning , its main effect was on Vasuki who was being used as rope .

Even Vasuki had consumed some of poison . His whole body was bruised

 because of excessive churning .

But he didn't complain nor he backed out . But he was getting tired . When God

 Shiva had come to consume the poison , Shiva observed Vasuki's condition .

God Shiva was impressed with Vasuki's devotion and wore him around his neck .

 He assured Vasuki that once amrit has come out then the vapours of amrit will

 help him to get back his lost strength .

Vasuki was humbled by the words of God Shiva .

  

This is the reason why we find snake around God Shiva's neck.