I always attributed it to ego, which is what many things come down to being driven/caused by. Women are naturally competitive with other women in a never-ending game of who is more desirable. A woman seeing a man who is 'spoken for' (dating/engaged/married) means he has found a desirable woman as a mate and that woman is not her. Her ego makes her feel like she needs to prove that she is more desirable than the woman he has and tries to get him to ditch/leave his woman for her. But she doesn't really want him, she just needs to prove that she is more desirable than the other woman. The more desirable the man (wealth, power, security), the more she needs to prove that she could have him, if she wanted him, and this other woman must see that her man found her to be more desirable. Him attempting to leave his woman for her fulfills her ego and she has 'won'. But she no longer wants him and he is left without either. The man wasn't the prize, her reinforcing her ego that she is more desirable than another woman is the prize. The more desirable the man that she has lured from another woman, the more inflated her ego becomes.