In Mary Shelley's Frankenstein, Victor decides (in chapter 18 of the eText provided by eNotes) that his must return to England. Prior to this decision, Victor has met with his creature. After hearing the tale of the creature, Victor hears what the creature wants from him: the creature desires a mate. Victor only agrees to create the mate if the creature promises to leave all lands inhabited my mankind. Based upon this agreement, Victor agrees.
After returning home, Victor realizes that he cannot create the mate for the creature in Geneva. Instead, Victor realizes that his work needs to be completed far from his home and family (given his fears that his family may be harmed and that people will come to know that he is responsible for the creation of the creatures).
Victor, contemplating this voyage, states the following:
I had heard of some discoveries having been made by an English philosopher, the knowledge of which was material to my success.
I had an insurmountable aversion to the idea of engaging myself in my loathsome task in my father's house, while in habits of familiar intercourse with those I loved. I knew that a thousand fearful accidents might occur, the slightest of which would disclose a tale to thrill all connected with me with horror. I was aware also that I should often lose all self-command, all capacity of hiding the harrowing sensations that would possess me during the progress of my unearthly occupation.
Therefore, it is in chapter eighteen where Victor details why he must go to England to create the mate for the creature. Essentially, Victor wishes to go to England in order to keep all he loves safe, people from correlating the creature with Victor, and from his family seeing how his own mental and physical attribute deteriorate.