Why does more than one story arc make a novel interesting?

Novels (as with any art form) are successful when they speak to the real lived experiences of readers. If you think about the life of even the most ordinary person, it is connected with other lives; stories branch out into other places and connect with unknown people. This connectivity means that we are all familiar with the relationship between different stories, and how to use one to understand another. As we experience this in life, so too do we desire it in literature. Multiple story arcs not only give us a greater volume of material to appreciate, but also resonate with the patterns of our own lives, the many overlapping and undulating stories within.
There are innumerable instructive quotes on the joys and benefits of reading books. One particular quote, however, offers some explanation as to why multiple narratives within a work are particularly enjoyable:
“A great book should leave you with many experiences, and slightly exhausted at the end. You live several lives while reading.”
- William Styron, from an interview in Writers at Work, First Series (1958), edited by George Plimpton.