Why do women obsess over men they'll never have?

Is it just me, or do women just obsess over men they can't have? Men tend to be very straightforward and blunt about their motives, EVEN when they're trying to be subtle. Most men give up after they've realized that the chase will ultimately lead them nowhere, whereas women only strengthen their determination in getting what it is that they naturally cannot have. For example, there are these two girls at a local coffee shop who both have a thing for my boyfriend. I'm confident enough not to really care about the situation as my boyfriend thinks they're both highly unattractive inside and out, but more importantly, because he loves me a lot. My main question is in regards to the way they connive and calculate their way into his line of sight. It's truly pathetic. I mean, they'll blush and jump at the chance at saying hello to him but purposefully and blatantly ignore me. It brings a tear to a woman's eye when her man is so lusted after by these lonely, emotional messes for women. :)