One of the themes throughout Frankenstein is isolation and loneliness. We see this in the three main men in the novel.
From his letters to his sister, we see the Robert Walton has separated himself from other men. First, he's on a ship far away from anyone he knows. He considers himself above his crew, and so he spends his time separate from them. As he writes to her, he wishes he had a friend on the ship to talk with. Soon after his crew rescues Victor, he begins to talk with him and "love him like a brother." In this stranger, he hopes the he has finally found his friend. As he says in the fourth letter:
I said in one of my letters, my dear Margaret, that I should find no friend on the wide ocean; yet I have found a man who, before his spirit had been broken by misery, I should have been happy to have possessed as the brother of my heart.
Victor also exhibits loneliness. He alienates himself from society as he builds his monster and then again after he creates him. As Victor tells Walton, as a child he was a loner. Aside from his friend, Henry, he spent most of his time with his studies.
My temper was sometimes violent, and my passions vehement; but by some law in my temperature they were turned not towards childish pursuits, but to an eager desire to learn, and not to learn all things indiscriminately. I confess that neither the structure of languages, nor the code of governments, nor the politics of various states possessed attractions for me. It was the secrets of heaven and earth that I desired to learn; and whether it was the outward substance of things, or the inner spirit of nature and the mysterious soul of man that occupied me, still my inquiries were directed to the metaphysical, or, in it highest sense, the physical secrets of the world.
His self imposed isolation again is devoted to his studies, just as Walton's isolation is devoted to his expedition. After the monster begin killing, he again isolates himself on the lake. On his night sails, he contemplates suicide by drowning himself in the lake.
The monster has perhaps the saddest isolation. Self imposed isolation for him is because he feels that he does not fit in. The first man who sees him, a shepherd, screams at his appearance. So, once his creator flees from him, he flees to the mountains to live alone. It is only once he observes the De Lacey's family and their interactions. By watching them he sees what he is missing and devises the plan to make his creator create again, but this time he wants a woman, or partner.