Is it true love or hunger for power that drives them to go all out for the same man?
Most women are highly competitive by nature, and seem that men have an innate ability to bring out their most competitive streak without even meaning to. Although some women will gracefully bow out of they see that the man they have had their eye on is already involved with other woman, others will see this as a challenge that they can't help but take on. In fact, for some, it renders the man in question even more desirable. But why is this? Why do women compete for men? To find answer for this question I have referred 'Ask Diana' at Online Dating Guide/Service.

Love:
The most obvious and least offensive reason is that they're deeply in love. If a woman truly believes that a certain man is 'the one' and they are meant to be together, she may very well stop at nothing to ensure that they end up together. The fact that he is involved with someone else means nothing to her because, in her mind, he is with the wrong person (i.e. not her), he just doesn't it yet.
She may try to seek out the other woman's flows, insecurities and weaknesses, and exploit these to her advantage by bringing them to the man's attention, thinking that making her look bad will make herself look good by default. She may even go as far as insulting or threatening the other woman, thinking that this will help her cause.
The truth is that while the man may be flattered that another woman is interested in him, such tactics will only make the interested party look disrespectful and aggressive, and will not help her cause at all. True love is not something that can be found using hateful or underhanded means.

Power:
An uglier side of female competition has nothing to do with love and everything to do with power. Unfortunately, there are many power-hungry women who would derive an unhealthy amount of satisfaction by taking away other woman's man. That's right: another woman's loss at their hands would actually bring them joy.
More unsettling still is the fact the power-seeking woman may not even like the guy she is fighting for. And the worst part is that the guy in question will probably be the last to know. He may interpret her intense flirting as a sign of genuine interest, not realizing that he is merely a means to an end; her feeling of power.
A good way to tell whether the woman is genuinely interested or just hungry for power is to see how her reaction to you changes(if at all) depending on whether or not you are in a relationship with someone else. If she never displayed the least amount of interest when your relationship status was undetermined and was all over you once she discovered that you had a girlfriend, you may want to question why?

Forbidden Fruit:
The longing for someone who is off-limits is what draws other women to compete for a man. The fact that they can't have certain man is precisely what makes them want him. Quite simply, they want what they can't have.
Many factors, including wealth, looks and the fact that the man is involved, can make a man seem like forbidden fruit to woman. Just hinting that you are involved may make some woman want you even more. But do not forget that this action can have exactly the opposite effect on the few women left who actually respect other people and their relationship.

A Few Good Men:
Another reason woman may vie for a man's attention is that they believe all the good guys are taken, meaning that any man that is involved with woman or being pursued must have it going. If a man is single, women assume there must be a good reason. This reasoning makes for a vicious cycle in which those who are singles remain so, and those who are attached often end up with inappropriate attention being lavished upon them.
On a lighter note, the good thing about this peculiar female reasoning is that guys just try to surround themselves with as many women as possible whenever they go out, as this will make them more appealing to women in general.