This is an amazingly true quote. Enthusiasm is essential for the creation of high quality or innovative code. As soon as it is gone, either the person that lost it abandons the project or that person will start writing band-aid style code that minimally passes the requirements.
Trusting people to come up with good work, but monitoring their output and helping where needed, can help to keep someone in the flow. It also helps to make people feel they are part of the team and that they actually get something for their efforts.