In Frankenstein, why does Victor Frankenstein choose to create the monster?
As a child, Victor dreams of the
glory [that] would attend the discovery if [he] could banish disease from the human frame and render man invulnerable to any but a violent death!
He starts to consider how wonderful it would be if he could manage to make human beings resistant to illness. As a young man, just about to leave for college, Victor is deeply affected by the illness of Elizabeth Lavenza, a close family friend, and the death of his beloved mother. He tells Captain Walton, of her passing,
It is so long before the mind can persuade itself that she whom we saw every day [...] can have departed forever—that the [...] sound of a voice so familiar and dear to the ear can be hushed, never more to be heard.
After his mother's death, the dream of rendering humanity invulnerable to disease takes on a new urgency, and Victor's desire to eradicate the effects of illnesses like scarlet fever (the sickness that took his mother and nearly took his friend) grows. It's unclear what the link is between reanimating dead body parts into a new person and the goal of making humanity disease-resistant, but Victor seems to see one, and this is one reason he chooses to make his creature. However, he is also very interested in attaining personal "glory."
Further, Victor is driven to scientific extravagance after being humiliated by Professor Krempe. He becomes disappointed by the realities of modern science when compared to the old pseudoscientists. He says,
[...] I had a contempt for the uses of modern natural philosophy. It was very different when the masters of the science sought immortality and power; such views, although futile, were grand; but now the scene was changed. The ambition of the inquirer seemed to limit itself to the annihilation of those visions on which my interest in science was chiefly founded. I was required to exchange chimeras of boundless grandeur for realities of little worth.
Professor Waldman takes Victor under his wing, in order to educate him in the ways of what is actually possible with modern science, and Victor sets out to do something that no one has ever done before: to create life as a god might.
In Victor Frankenstein we find a character who is deeply flawed despite of his great academic gifts. One of his major flaws as a character is the tendency to think that his beliefs are equivalent to rules and, as such, he must follow them.
When Victor loses his mother his emotions falsely lead him to suppose that he could have some control over the fate of people's lives if only he had the proper formula. This is the extent to which Victor's ideas are, in his opinion, norms. As a result, Victor summed up all of his intellectual and research skills to obsessively find that proper formula that would create life.
Hence, it is the combination of Victor's own sense of grandiosity, his obsession with creating life, and the many new discoveries taking place around him involving chemistry and electricity, that made Victor suppose that he could use all those resources to conduct the ultimate science project: the creation of life itself.
Victor also surely has a strong ambition to play God, in a sense. He wants to push himself to see exactly what he is scientifically capable of, aiming to be the first to ever bring a creature to life (Shelley may well have been writing in reaction to concerns in her time that science may be going too far with experiments on re-animating living beings through electrocution via lightning). He is so blinded by this ambition that he does not even notice how hideous his creature is until it comes to life. Since Shelley describes the creature as being quite, quite gruesome, it is clear that Victor's ambition was immensely strong so as to blind him from this fact.