If you are in a relationship or you want to be in one, or the girl you are seeing is good enough to make your family in the future you think then it is supposed to know everything about each other. In this case men are quite open to share their previous relationships and every aspects of their lives which they can't share with everyone. But girls do not tend to share most of the things with their boyfriends or their husbands. It is very difficult to understand women and their nature. Girls try to hide most of their early relationships and want to convince their latest boyfriends that he is the first boy she has ever dated or been in an relationship.
This is not the only thing to worry about. The problem becomes severe when girls do not say their previous boyfriends that she want to break up clearly. All the previous boys are kept as an option in a way if the relationship doesn't work with one. In this way they feel more secure.
So, leaving some of the exceptions, if you think you are the only man who is 24/7 around your girl's head then you are completely wrong. There are lots of other boys which she thinks of. Just the difference is you reside in the top of priority list.