I would say that this girl doubting her boyfriend because of some serious underlying beliefs. Most probably, she does not believe that she deserves that much love or devotion. Low self-worth is more common than we think, and it hides in many small details, like refusing to be loved that much, for example.
The other belief which is obvious to me, is that she lacks trust in people, or maybe only in men. She believes that he is too good to be true, and that he is acting so well, because he is hiding something from her. Most probably, she thinks he is cheating on her.
I can bet any amount of money that she will discover something that will validate her doubts pretty soon.
I see light blog
The image is a courtesy of "Daily Drop Cap by Jessica Hische"