1. He has a crush on this girl, or has fallen in love with her. He may want to protect her and care about her life and health to show his value and trust.
This boy may be a friend or acquaintance of this girl. He may think that this girl needs extra support and attention, because she may be fragile or lack of self-protection.
This boy may be controlling or jealous. He may think that this girl should belong to him completely, or he should not be in contact with other men. This kind of behavior may make girls feel that their privacy is violated and uncomfortable.
The boy may have an intention. He may want to get some benefits from this girl, such as hoping that she can help him finish something, or trying to steal her resources and opportunities. This kind of behavior is usually immoral or illegal.
This boy may lack self-confidence and security, and may be afraid of being separated from this girl, because he may not know how to deal with his emotions and anxiety. This kind of behavior may make girls feel restricted and uncomfortable.
In short, there are many possible reasons for a boy to follow a girl, both good and bad. If girls feel uncomfortable or their privacy is violated, they should communicate with boys in time to express their feelings and needs. If boys are really interested, girls should seek help in time to protect their safety and rights.