Why did Victor's father marry Victor's mother in "Frankenstein?"
The story of Victor's mother and father is important because it sets up the idea of duty, something that Victor will fail in later in in the book. Victor's mother was the daughter of a man named Beaufort, who had been a business partner and friend of Victor's father. Beaufort falls on hard times, becoming very poor and sick. Victor's father seeks out Beaufort, and finds him and Caroline (his daughter) just as his old friend has died.
In concern and compassion, and in duty, Victor's father takes Caroline under his wing, placing her with a relative to ensure that she was well cared for. Victor just tells us that two years later they married - we can assume that as she lived with the Frankenstein relatives, Victor's father visited with and fell in love with her.
There was a considerable difference between the ages of my parents, but this circumstance seemed to unite them only closer in bonds of devoted affection. There was a sense of justice in my father's upright mind, which rendered it necessary that he should approve highly to love strongly. ...There was a show of gratitude and worship in his attachment to my mother, differing wholly from the doting fondness of age, for it was inspired by reverence for her virtues, and a desire to be the means of, in some degree, recompensing her for the sorrows she had endured...
The above quote suggests that the affection between them was genuine. It also shows that "justice" is part of what pushed Victor's father towards marriage, added to an appreciation for his mothers "virtues" and for her hardships.