Because she likes you but she's shy to admit it-- she's spying/looking at you just the way your doing it to her but she doesn't want you to know it--shes playong hard to get--shes mad at you--you...
Because he doesn't want you think he was starring at you... or he feels awkard/embarassed that you caught him looking at you...
Answer .
Because he doesn't want you think he was starring at you......