Answer by Justine Musk:
Money is rarely just money. Sometimes it stands in for love or self-esteem or freedom or a sense of control over your destiny (especially if you lacked these things in childhood). Sometimes it is a way of controlling others, including family members, because you don't know how to connect to them in any other way (the now-deceased patriarch of one of the famous American families was particularly notorious for this). Money can also serve as a scorecard to indicate how well you are doing, the impact you are having, if you are winning. Sometimes it's not so much the money that matters, but the win is everything — particularly when you are invested heart and soul in your mission.
Edited to clarify: When I say money serves as a scorecard, I mean that money in and of itself is not a primary motivation. Money signals that value is being created and all systems are operating well.