Often, they become aware that they "want to make a difference."
The problem is that making a difference isn't easy because so many things that would appear to make a difference don't. In fact, not only do those things not make a difference, they actually, directly or indirectly, make things worse.
BTW, there is an exception to the above. Sometimes, right after people have made a lot of money, they realize that money isn't enough. They realize that you can have a huge amount of money and still have no discernable power. So they set out to gain a lot of power.
Sometimes the desire to make a difference and the desire to increase one's power collide. This is really dangerous because you become a person who is self-righteous in your effort to gain power. And you yield such power self-righteously. In fact, in your mind, you become a mini-god (or is that "demi-god"?).
Wealth, power, righteousness.
I think we might be able to draw the conclusion that Bill Gates misspends his wealth even more badly than the government would misspend it - but that still doesn’t make a valid argument for increased taxation.