How can I introduce an essay which argues that Victor Frankenstein’s abundance of friendship and love leads him to be overly confident in his abilities, while the creature’s lack of the same prevents him from benefiting from his greater human potential?

A really nice way to begin an opening paragraph is by introducing a relevant quotation or idea and then linking it to your topic. One quote that seems very applicable to your topic comes from Elie Wiesel, another author: "Friendship marks a life even more deeply than love. Love risks degenerating into obsession, friendship is never anything but sharing."
Victor, of course, has wonderful friends in Henry and Elizabeth, and they help him to feel better when he's down; Henry even nurses Victor back to help when he falls dangerously ill after the completion of his experiment.
The creature, on the other hand, is utterly friendless—even his creator abandons him. Though he desperately tries to reach out to the DeLaceys, they also run in terror from him. Finally, he attempts to kidnap a child in order to educate the child as his companion and friend, but when he learns that this child is Victor's brother, he kills him in order to cause Victor pain. It is clear that having friendship, real and true companionship is vital, more vital than even romantic love. Because he has no one to share his life with, the creature is miserable.
You say that you plan to argue in your thesis that Victor's abundance of friends and loved ones makes him overconfident, and the creature's lack of friends prevents him from reaching his full potential. You might point out that it is only when Victor is left alone, friendless in Ingolstadt, that he pursues his education exclusively, and this practice leads to his grave error in seeking to create life. He says,

In the university, whither I was going, I must form my own friends, and be my own protector. My life had hitherto been remarkably secluded and domestic; and this had given me the invincible repugnance to new countenances. . . I believe myself totally unfitted for the company of strangers.

In other words, because of his wealth of friends at home, Victor felt comfortable NOT making friends at school. Instead, he spends all his time working on his education, and then on his experiment—his attempt to create life—and this is where his major trouble started.
The creature, on the other hand, doesn't choose solitude, but rather, has it inflicted on him, like a punishment. He tells Victor,

"My vices are the children of a forced solitude that I abhor; and my virtues will necessarily arise when I live in communion with an equal. I shall feel the affections of a sensitive being, and become linked to the chain of existence and events, from which I am now excluded."

Victor ultimately refuses his creature's request for a companion, and thus the creature turns all of his efforts into exacting his revenge on Victor. Rather than be able to use his powers for good, to assist humanity, to further our lives, the creature's life becomes consumed by misery and malice. Any idea that he could benefit the race of humanity is lost because he is so lonely.