Which one of these two is correct? Is it necessary to write 'you are' instead of only 'you'?
In a sentence with "I" vs. "you," I can see where adding or subtracting the "do" is a matter of style choice.
But in the sentence "Nobody knows your life better than you" there is room for ambiguity.
Without adding the "do," the meaning might also become "Nobody knows your life better than (they know) you."
Adding the "do," it becomes clearer that the meaning is "Nobody knows your life better than you (know your life) do."