Frankenstein’s strongest motivation for creating life was his desire to play God and to be remembered as a great scientist. In opting to create life, Frankenstein was acting out an extremely selfish desire: he wanted to be remembered as the man who created life.
He was also motivated by his need to prove to his family and society that he was not a monster, but a genius. Frankenstein had been shunned by society all his life because of how hideous he looked and because he was seen as a freak of nature. He wanted badly to prove them wrong by demonstrating that he could create life in spite of their rejection of him.
Additionally, Frankenstein was motivated by his own curiosity and desire to understand the mysteries of life and death. He was consumed with questions about what it meant for something to be alive, whether it could think like humans do, if there was some sort of soul inside it—and these questions drove him forward into creating life despite all the risks involved with such an undertaking.
Finally, Frankenstein was motivated by his hope that his creation would be a companion who would never leave him. By creating something new out of nothingness (or close enough), Frankenstein hoped that he would have someone who would always love him unconditionally—someone who would always stay with him no matter.
