Why did Dr. Frankenstein create his monster?

The short answer to your question might be this: although Victor Frankenstein claimed to be creating his monster for the betterment of humankind, it's more likely that he did so out of arrogance, or out of a desire to become like God.

As a young man, Victor's interests lie in science, chemistry, and of the balance and contrasts between life and death. While a university student, Victor becomes obsessed with the idea of creating life out of inanimate objects and starts considering how to do so. Victor thought he was doing a service to humanity by creating a "new human."

At one point, Victor says, "A new species would bless me as its creator and source; many happy and excellent natures would owe their being to me. I might in process of time (although I now found it impossible) renew life where death had apparently devoted the body to corruption." This quote shows insight into Victor's motivation: he wants to figure out how to cheat death, and he had allowed himself to be overcome by ego. If successful, Victor would be revered by the creature(s) he creates and his creations would make Victor a human god, or so he thought.

Of course, after stitching together various parts of human corpses to create his new being, Victor succeeds in reanimating the dead, but is immediately repulsed by his creation and its "ugliness." Victor falls into a deep depression and shuns his creation, perhaps because now that he's conquered death, his replacement as God is complete. Victor is similar to Goethe's Faust character who went on a quest for knowledge, made a deal with the devil, and is rescued by God. Unfortunately, Victor does not have the benefit of divine intervention. Unlike Faust, Victor knows he will not be saved and instead will perish without redemption.