Others equate money with self-worth. Money can mean power. In a relationship where a woman earns more than a man, this can be a challenge. Whoever has more power can be undermined by the person who doesn't. It takes a very strong sense of self-esteem to be with a male or female who makes a disparate amount more.
Money can be about revenge.
She's mad that he works late, so she goes shopping. With so many feelings tangled in with money, it's sometimes hard for people to talk about it without getting emotional or even irrational. But discussing money on a regular basis is a way to avoid arguments. Trouble is, lots of people don't like to talk about money. It was easier to talk about sex than it was to talk about money.