It means this guy is either a crush/ or boyfriend who she is falling for but she has been hurt or thinks she is gonna be hurt emotionally later by this guy, so she is putting her guards up and not trying to fall in love with him as she know she will be hurt. She is afraid of falling in love so she's trying to not to fall in love even though she is, but she is trying to prevent it.
It means this guy is either a crush/ or boyfriend who she is falling for but she has been hurt or thinks she is gonna be hurt emotionally later by this guy, so she is putting her guards up and not trying to fall in love with him as she know she will be hurt. She is afraid of falling in love so she's trying to not to fall in love even though she is, but she is trying to prevent it.