Sometimes people will say something negative about themselves because they're hoping you will say something positive, instead. I used to do the same thing. Whenever I was feeling insecure about my looks or weight, I would say something negative about myself, because my husband would tell me it wasn't true and would give me compliments that would boost my self-confidence. I don't do that anymore, because it's not a very healthy way to be confident in yourself, but it certainly was effective. Do you think maybe your girlfriend is just feeling insecure about herself? I would talk to her about that to see if maybe it's something she'd like to work on in herself. You can also try to give her honest compliments on a regular basis. It's hard for a girl (or guy!) to feel unattractive if her loved one is telling her twice a day that her shirt looks good on her, or he loves her laugh, or he thinks she's pretty.