For what reason does shakespeare commonly choose to write in iambic pentameter?

It was the way the upper class talked and i guess like any one else he wanted a sort of rhythm to his poem XD
Good luck hope this helps 

Shakespeare, like many poets through the years, use iambic pentameter because it gives the poem a certain flow and actually defines it as poetry. Shakespeare and iambic pentameter go together hand in hand because he used the form so often. Look at any sonnet he's written and you'll instantly see examples of it.