The main reason most girls are jealous is low self-esteem, insecurities, and lack of trust. Anyone can experience jealousy. Maybe she had a bad experience with a past boyfriend. Perhaps he cheated on her and she was really hurt over it. That would certainly cause problems of self-esteem.
One main reason girls tend to get jealous is over other girls, particularly ex girlfriends. They are worried that other girls might have something that they do not. Your girlfriend may feel that you could find other girls prettier, funnier, or smarter than she is. She may also be wondering if you still have feelings for your ex. All of these thoughts will lead to the fear that she could lose you.
*So boys.. do understands us.