Agile Manifesto: Principle #4
"Business people and developers must work together daily throughout the project."
Unfortunately, this principle can be hard to swallow for some.
Comments like, "I've already told the developers what I want, why can't they just go build it?" or "I'm too busy with my day job to support the developers." are not uncommon.
This daily interaction can be a new way of working for many people. Generally, once people start working this way, they start to appreciate why it's necessary. They recognize that things change much faster than they realize and that constant communication is necessary in order to adapt.
An often understated side effect of this principle is that it starts to establish trust.