Why do we always seem to want what we can't have?

So, apparently that's my life - striving for things I'll never get.
Take one of the many examples. I've been loving this guy for 3 and a half years, he's amazing and makes me SO happy. But he's kind of with someone else now and that kind of kills me. But I want him more than I ever did. Why? Well I think it's because I've realized how much I care about him. Although it could be because I can't have him. Twist it around, he's wanted me for so long, I was never really available - now I put myself out there and I am, and he doesn't want me anymore. Why? Cause he can have me. Afterall, what fun is a girl who will give it all.
Rejection sucks and I seem to go through strings of it at a time. Which of course makes me question and second guess everything I do. There's just one thing that I want and that is just to be loved. Is there something wrong with me that guys don't want to stick around, there all up for the good stuff and then out of here. Whatever I don't really care, want me for whatever, I'm used to it.
This guy, the one I love, I've never cried so much before. My undereyes actually sting from crying so much, they're dry. He says he loves me. But he's dating some other girl, and he wants to see what that'll bring, but he doesn't want to lose me. So we're going to be friends and I'm going to stand by him and there's this possibility that he might fall in love with her. Could that hurt anymore? He's my rock. Is that pathetic or does everyone have one? He's the one person who's been there through thick and thin. We've broken up before and he's still been there to comfort me. But now, it's like he's stabbing me and twisting the knife around. How can I possibly deal with the fact that he could be hugging, kissing and touching a girl the way he hugs, kisses and touches me. I can't bear it, and I've told him, and I've cried in front of him and he's seen how much it hurts me - but this is what he needs. What do I do? How do I deal? Do I let this happen, because I've tried to walk away - and if I do - that's it, no more, no more hope, no more talking, no more anything. So I'm suppose to cling to a hope that one day maybe he'll love me enough and trust me enough to want to be with me. But in the meantime I'm left nursing my aching heart and trying to smile through each day. So here's the moral of the story: when you have someone you love and you're going through a rough time - don't hurt them, talk it out and work on it. Don't screw with their heads don't risk someone you love for someone that you might like or might even love. Because it's not worth it. When you love someone it's this beautiful thing and screwing it up for some one night stand or some jerk who thinks he's hot shit isn't worth it.

So here I am, trying to move on from my first love. But I guess the first cut is the deepest.

So here I am still waiting for my Knight in shining armour.... one day.