In Frankenstein, why does Victor Frankenstein choose to create the monster?
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.
Victor Frankenstein decided to create a monster as a result of loosing his mother. When his mother died, he wanted to prevent anyone from ever having to lose a loved one. At her grave he said, "No one need ever die. I'll make sure of that".
Hope this helps.