I think everything has a foundation - so also a person's enthusiasm.
How much the world need people with enthusiasm! They move the grey masses.
I also think there is something to what Frank Bettger states in his classic book "How I raised myself from failure to success in selling"
He says: "To become enthusiastic - act enthusiastic". He even says: "Force yourself to act entusiastic and you'll become enthusiastic" and
"Enthusiasm will help you overcome fear, become more successful in business, make more money, enjoy a healthier, richer and happier life".
So Frank Bettger talks about enthusiasm as a very needed trait - and if you don't possess it, how you can make it part of your constituent. This can seem as an outside - in approach. And in a way it is. But on the other hand - what makes a person apply the rule: "Act entusiastic - and you'll become entusiastic". It is the dream, the need, the vision to be an enthusiastic person and that beeing an enthusiatic person is necessary for me to reach my results of moving this team, be a person of influence, move myself to a higher level, etc. It triggers more endorphins - a happier attitude.
Sometimes we have to throw ourself (our mind) over the bar and the rest (our body) will follow.
However everything is built on a foundation - and I believe a person's enthusiasm has a foundation even if it is forced to start with. And you might be right that the right foundation or a better word for extended enthusiasm is Passion.
We come to the real reason why people are doing what they are doing. Are we just in the motions doing what others tell us to do, or are we in charge of our own life, with real purpose, constantly pursuing worthwhile goals, magnifying our gifts, etc. The real poverty of this earth is that many of us do not take on this endeavor, for different reasons.
Passion is emotional power and is the best fuel for any endeavor.
Skip Ross says: "Get a clear mental picture, fill it with emotions and hold on to it"
Michael Gerber says:
"...Once you have a picture of how you want your life to be, and you come to the realization that it is more than just things to have and things to do, once you realize that what you and I really want is to have the room, the openess, to expand, to grow, to be more of ourselves, whatever that means, and to find out what that means is what's most important to us, once you see that, you can then turn to the business that's going to help you get there, you can then turn to the development of your Strategic Objective.." from the book "Why Most Small Businesses Don't Work and What to Do About It"

Eleanor Roosevelt said: "The future belongs to those who believe in the beauty of their dreams"

Perhaps it is right that sustained enthusiasm requires or is passion. But enthusiasm (perhaps we can call it the outward expression of passion) is real important, and I need it, you need it and the world needs it.
"Nothing great was ever achieved without enthusiasm"

The big question is - how do people get filled with passion (and enthusiasm)?

Rolf I

A life without passion is a life half lived.