Why When I Have The Guy I Want, I Always Want Someone New?

Your emotional development seems that of a 13-year old girl.  I suspect that you see the ability to form interpersonal relationships as an indication of your worth and a romantic relationship as a solution.  A rewarding experience would be for you to see how much you can add to someone else's life instead of how much you can take.  My advice on how you can hold your focus is to keep track of how much you contribute.  Set a goal.  Think carefully because not everything you think of at first will be a positive contribution.
 
I could be all wrong.  You might just be surrounded by dweebs and need to find a better crowd to associate with.

The grass always seems greener on the other side of the hill. When you like a guy, he has all these imaginary qualities, that you expect him to have. When you get close to him, he does not live up to your expectations. Your expectations may be a little unrealistic. There are no perfect men or women out there. However, there are good men and good women. When you get close to someone, do you focus on their faults, or on their assets? I agree with Slammer, you get out of a relationship, what you put into that relationship. If the first time a guy fails to live up to your expectations, you start looking for another, you may end up being a lonely person. When you care about someone, you learn to live with their little defects of character. I am sure you have some unpleasant habits of your own. It takes time and patience to 'fine tune' a relationship, rethink your expectations and think of 'the long run'. A man who truly loves you and is responsible and hardworking, beats out Mr.' Good Looking', the world owes him a living. When you are truly mature, you will find that out.

I agree with Pencil and Justness.  I was in that same situation.  Yes and sometimes you do think that the grass is greener on the other side.  I use to go out with all these sexy guys but the only thing about sexy guys is that they are every bodies man and sometimes that's all they have is the bodies.  Well, I eventually got married to a man which was the total opposite of all the guys that I use to date but I knew that he was the one I wanted to spend my live with.  Nobody could believe it but looks are not everything.  We were together for 19 years.  He was a wonderful man but I got board and the grass was greener on the other side.  To make a long story short.  I lost my man and the other guy, well as soon as he found out that I'm divorced he developed wings and flu away.  So now I end up alone.  Well my husband and I still love edge other very much and we don't see other people but its gone take a long time for him to trust me again and enough to take me back.  Well, I'm doing everythings in my power now to win my man back because the longer I wait the more change there is that he can meet someone ells and then I've lost him forever.  Now the lesson in this is.  If you are young and still dating then good for you.  Its good to explore your horizons but if you are involved in a relationship and you cant stick to one man then you got a serious problem because you must remember you are not getting younger and your looks if that is the case will not last forever and you are going to end up all alone if this is the case.

I very agree with pencil and slammer. I believe that we human will always look for satisfactions to fit our expectations. We wont stop until we find it or realize that we are expecting too much and need to eliminate some.
In your situation I'm not sure if you meant as in relationship wise, just sex, or both. See when we see someone we think fits our category we want them badly until we open the book and read it and it turn out boring. You need to find out what you are longing for? What is your desire? Until then you'll never stop doing what you are doing.
Take the time to learn how to love what you have. If you keep doing what you're doing you might just end up like justness. She's lucky to have him waiting for her. What if he didn't wait? What if you lost him forever? I'd say rest the dating for now and find your inner self first.

You can read the comment in pencil answer i think my story may help you to say your relationship by different way,,, thanks