There are a few reasons why Victor Frankenstein chooses to create a monster.
First, we can say that Victor Frankenstein’s scientific mind led him to create life. More specifically, as he was studying the chemical decay of living beings, he gained knowledge of creation and life. He pushed this knowledge to it logical end.
Second, we can say that there was also an emotion component. After his mother died, he was motivated to cure the problem of death.
Finally, we can say that hubris got the best of Victor Frankenstein. He had a god complex. To put it another way, Frankenstein had a ruthless desire to obtain forbidden knowledge. This is confirmed when the monster calls himself, Adam, ("the Adam of your labors").
One of the most famous quotes in the book touches upon this theme:
"Did I request thee, Maker, from my clay
To mould me Man, did I solicit thee
From darkness to promote me?"
The main "protagonist" in Mary Shelley's classic of gothic literature, Frankenstein, is a man driven to learn and to experiment. That is all well and good, but, unfortunately for Victor Frankenstein, the particular subject of his obsession is the reanimation of dead tissue. In short, Victor is determined to prove that the dead can be brought back to life. Shelley's novel is more than just a horror story. It is a philosophical contemplation of the nature of life and of the ethics of playing God.
Victor Frankenstein's motivation in creating the monster, or, as he will put it, "the wretch," had its roots in his childhood fascination with science. Early in her novel, Shelley's protagonist describes his discovery of an ancient text that spurs his interest in both science and literature. It is his father's condemnation of that ancient text, referring to it as "sad trash," however, that leads Victor down the path of scientific investigation--a journey that will climax with the destruction of all he holds dear. Rather than follow his father's advice, young Victor proceeds to read this ancient text, that of Cornelius Agrippa. As Victor explains the consequences of this youthful decision:
"If, instead of this remark, my father had taken the pains, to explain to me, that the principles of Agrippa had been entirely exploded, and that a modern system of science had been introduced, which possessed much greater powers than the ancient, because the powers of the latter were chimerical, while those of the former were real and practical; under such circumstances, I should certainly have thrown Agrippa aside, and, with my imagination warmed as it was, should probably have applied myself to the more rational theory of chemistry which has resulted from modern discoveries. It is even possible, that the train of my ideas would never have received the fatal impulse that led to my ruin. But the cursory glance my father had taken of my volume by no means assured me that he was acquainted with its contents; and I continued to read with the greatest avidity."
This is no throw-away passage in Shelley's novel; on the contrary, it is the beginning of that ominous chain of events and involves also the death of Victor's beloved mother--an event that instills in the young man a need to understand the line that divides life from death.
Victor will grow and mature, and his fascination with fields of science outside the bounds of the conventional takes root with tragic consequences. Victor's interest in science, in particular in the medical sciences, is admirable, but his fascination with the unconventional is not, as he acknowledges early in his confession before Robert Walton:
"My dreams were therefore undisturbed by reality; and I entered with the greatest diligence into the search of the philosopher's stone and the elixir of life."
While Victor's fascination with particular discussions of the natural sciences sets him upon his path toward the creation of the monster, it is the countervailing views of M. Waldman, along with this learned professor's admirable demeanor, that further propels the now-university-student Victor along his journey of discovery. As Victor describes the effect on him of Waldman's guidance, he states, "From this day natural philosophy, and particularly chemistry in the most comprehensive sense of the term, became nearly my sole occupation." So, again, we have information directly related to his motivations for experimentation on reanimating dead tissue. Victor's studies lead him closer and closer to the fundamental questions of life and death--"Whence, I often asked myself, did the principle of life proceed"--and he continues with an unhealthy passion to study and to experiment.
Victor is driven to learn about life and to experiment with ways to reanimate dead tissue. Chapter III, from which the above quotes are taken, is replete with examples of text illuminating his motivations:
"To examine the causes of life, we must first have recourse to death. I became acquainted with the science of anatomy: but this was not sufficient; I must also observe the natural decay and corruption of the human body."
Victor Frankenstein is committed to the pursuit of knowledge. It is, however, his experimentation with living beings that proves catastrophic. As noted above, Frankenstein is a thoughtful meditation on the ethics of scientific experimentation. Chapters III and IV, however, also provide useful meditations on the ethics and morality of scientific experimentation when thorough consideration of consequences is absent.