Most women are highly competitive by nature, and it seems that men have an innate ability to bring out their most competitive streak--without even trying to. Although some women will gracefully bow out if they see that the guy they've had their eye on is already involved with another woman, others will see this as a challenge that they can't help but take on.
So, why do women compete for men? Quite simply, they want what they cannot have. Some women fight for a man's attention because they believe all the good guys are taken and that any man that is involved with a woman must have it going on. If a guy is single, women assume there must be a good reason.
The longing for someone who is off-limits is what draws other women to compete for a man, and a lot of women find a taken man intriguing and something that they just cannot pass up. The simple fact that they can't have a certain man is precisely what makes women want what's off limits.
If a competitive woman of this nature truly believes that a certain guy is the one for them 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 another woman means absolutely nothing to some women because, in their minds, he is with the wrong girl-- he just doesn't know it yet! She may try to seek out the other woman's flaws and weaknesses and exploit them to her advantage by bringing them to the man's attention, thinking that this will help her cause and make herself look good in the man's eyes.
Unfortunately, another side of female competition is dealing with power-hungry women who would actually get joy and satisfaction from taking away another woman's man. This type of power-hungry woman probably never displayed the least amount of interest in a man while he was single, but was all over him once she discovered that he had a girlfriend.
In this situation, a guy may interpret the power-hungry woman's intense flirting as a sign of genuine interest when in reality she may not even like him at all and is simply being used to satisfy her unhealthy need for power over another woman. A lot of women only date married men for this exact reason.
Remember, women want what other women have, and the more out of reach you are, the more appealing you are to them. So use this warped female reasoning to your advantage by surrounding yourself with as many quality women as possible when you go out, as this will make you virtually irresistible.