To good to be true is supposed to be something said for something positive but most of the time used as a negative but in this case her to good to be true is a positive it’s everything she could ever ask for it’s her fairy tale her happy ending it’s like when something great happens to you and you think this can’t be happening to me but then if she is using this as a negative then it’s doubt in her mind cause she not used to anybody being this way and feel like he is being so nice and doing all this things for her to try to get something out of her which it’s not going to work out just cause of this doubts and make him not even want to do anything for her anymore which she can be like why don’t you do this type of things anymore and push him away but there can be couple different meanings for it just depends on the person and the situation