Why Are People So Concerned About How Others Spend Their Money?

I'd hate to be at a blackjack table with the people who think how others spend their money doesn't affect them.

 Perhaps some people like the idea of having influenced somebody in how they spent big $$$ on a purchase?

Some people are overly concerned with 'fairness'. My brother is like that. Funny thing is that he used to say that same thing as a child: 'That's not fair!'. Sounds good at face value (the idea of fairness) but it can get out of hand really fast. 

I've never been concerned about what/how other people do...none of my business. As long as they're not harming themselves/others.

@sandthemall 

I've never been concerned about what/how other people do...none of my business. As long as they're not harming themselves/others.

Well said. A surprising amount of what we do harms others. That's the tricky bit.