How were differing “virtues” a cause for the outbreak of the American Civil War?
The "virtues" of the North and the South can be seen as a cause of the Civil War because they influenced the two section to see themselves as very different from one another.
The South believed that it had the virtues of a more traditional and aristocratic society. It believed that its people were connected to each other by personal ties (master to slave, rich patron to poorer client). It saw this as different from and superior to the more impersonal Northern society that was tied together only by crass economic need.
The North saw itself as the society of freedom. Its virtue was that all people had the right to do what they wanted with their lives. This made it a more individualistic society that emphasized liberty over community.
Because of these differences, each section believed that the other was different and, importantly, worse. This helped lead to the Civil War because it made the two sides feel that they had very little in common.