In my experience its cos deep down you dont like the person. Sexual arousal starts in the brain. Women like to be turned on in their head first, not just physically. When you meet some one you actually like, you will LOVE the feeling of their penis inside you and you WILL be able to feel it.
Anxiety can cause some of this. If you are very concerned about pleasing the other person or being a desirable girlfriend, or very self-conscious about performance, that can be a major buzz-kill. One way to tell is if you have orgasms when masturbating. If so, then there is nothing about your vagina and clitoris that is the problem, it has more to do with relaxing and getting excited. Do you get excited when reading sexy material in novels? That would be another way to tell.
I am not a woman, so I don't know for sure if it is normal, but I do not think it is. The good news is, sooner than later, there will be women or men with the health knowledge who will help you out. I hate to say it, because of how embarrassed or afraid you are to do it, but I think you should see a doctor. Please hang tight though, I just posted mainly so you can see that people do see and answer the questions people ask.