How can I write a good thesis about isolation in Frankenstein?
Isolation is essentially what makes Frankenstein's creature so miserable. It is because he has no companion, no one to love and be loved by, that he becomes violent and brutal. In the end, it is his complete isolation (after the death of the only "companion" he's ever had: his creator) that leads him to take his own life. Further, isolation is the condition he attempts to replicate for Victor in order to render Victor as miserable as he, and so this proves just how devastated he is by isolation.
Then, when Victor is alone in Ingolstadt, after the creation of the monster, he becomes violently ill and requires his friend, Henry Clerval, to help him return to health. Similarly, after Clerval's death, when Victor finds himself alone again, he becomes very ill and requires his father to nurse him back to health. Thus, the creature can survive his isolation but does not want to, and Victor doesn't actually seem to be able to survive isolation. Thus, you could claim that isolation is a life-threatening condition in this novel; a person simply cannot be happy and healthy when they exist in isolation.