As all other phenomenon sex is a great and natural phenomenon and no human being can avoid it in any case unless he or she is medically unfit. The attraction of boys towards girls and of girls towards boys is a natural thing and it cannot be avoided in any case. Now we come towards your question that why are boys always chasing girls all the times can be answered in the following way.
When a boy gets young and he starts feeling urge of sex, it's a natural phenomenon and we should not be amazed. In exposing expressions boys are bolder as compare to girls and they don't feel hesitant in expressing their love to girls. For this purpose they do many things either sensible or foolish. But in my opinion only those boys always chase girls who don't have any other goal in their life and they are idle. As they have plenty of free time and they perform this act. The boys with goals and aims, I don't say that they don't feel urge of sex but they stick to their goals and at last succeed in life. It is better to chase girls after getting success in life.
When a boy gets young and he starts feeling urge of sex, it's a natural phenomenon and we should not be amazed. In exposing expressions boys are bolder as compare to girls and they don't feel hesitant in expressing their love to girls. For this purpose they do many things either sensible or foolish. But in my opinion only those boys always chase girls who don't have any other goal in their life and they are idle. As they have plenty of free time and they perform this act. The boys with goals and aims, I don't say that they don't feel urge of sex but they stick to their goals and at last succeed in life. It is better to chase girls after getting success in life.