We see, in the novel, that isolation is bad for everyone, emotionally and sometimes physically, as well. Captain Walton tells his sister in one of his earliest letters,
You may deem me romantic, my dear sister, but I bitterly feel the want of a friend. I have no one near me, gentle yet courageous, possessed of a cultivated as well as of a capacious mind, whose tastes are like my own, to approve or amend my plans. How would such a friend repair the faults of your poor brother!
Walton is sad that he lacks a friend to whom he might tell his plans, his hopes; he wishes that he had someone with whom to commiserate, who might share his passions and interests, who could help to improve him as a person. He is lonely, and his loneliness causes him sorrow. When his crew finds Victor on the ice, Walton believes that this man could have been the friend for which he's been hoping, but he's aware that Victor is not long for this world.
Likewise, Victor suffers from his lack of companionship when he's in Ingolstadt, working on his project. He's been away from home for several years, and there is no one near him in whom he can confide or who can provide comfort and succor to him as he becomes more and more ill. He says, "Every night I was oppressed by a slow fever, and I became nervous to a most painful degree; the fall of a leaf startled me, and I shunned my fellow creatures as if I had been guilty of a crime." Finally, his best friend, Henry Clerval, arrives, and Victor credits him with nursing Victor back to health. In order to recover, then, Victor required a friendly soul who cared for him. Something similar happens later in the story when Victor's father comes to be with him in jail, after he's been accused of murdering Henry.
This actually seems to be one way to confirm the creature's humanity: he requires companionship, as does every other major character in the novel, in order to be healthy and happy.