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The Wikipedia page on the Monomyth has these statements in the summary at the top:

Campbell held that numerous myths from disparate times and regions share fundamental structures and stages,

and

Campbell argues that classic myths from many cultures follow this basic pattern.^{[citation needed]}

Unfortunately, the second one doesn't have a reference. However, I think there's a pretty good chance that this was Campbell's belief. Now, I'm wondering if he ever said anything about why so many myths follow this pattern. I'm not looking for anyone's opinion - I'm specifically looking for Campbell's opinion on this matter. Why is it that so many myths from all sorts of cultures follow the monomyth pattern?

The Wikipedia page on the Monomyth has these statements in the summary at the top:

Campbell held that numerous myths from disparate times and regions share fundamental structures and stages,

and

Campbell argues that classic myths from many cultures follow this basic pattern.

Unfortunately, the second one doesn't have a reference. However, I think there's a pretty good chance that this was Campbell's belief. Now, I'm wondering if he ever said anything about why so many myths follow this pattern. I'm not looking for anyone's opinion - I'm specifically looking for Campbell's opinion on this matter. 

Why is it that so many myths from all sorts of cultures follow the mono-myth pattern?