Part of why my relationship with my ex never worked out was because I always thought he was just too good for me. As I have mentioned earlier, he was everything I thought I wanted (handsome, intelligent, sarcastic etc etc). Additionally I initiated the contact with my ex and pushed the relationship in the beginning. I was also the first one to kiss him. Add such a progression of events to my lacking self-esteem and you get potentially poisonous mix.
See when you are convinced that someone is too good for you, you will see rejection everywhere. You will analyze what they say in a manner which is predisposed to finding something negative, you will perceive their actions as a sign that they do not want you and you will try to act not the way you would act with your friends, but you will try to project the kind of image you think your lover might want you to be. Furthermore, you keep hoping that your love interest will help you to finally feel better about yourself, but the problem is, in no way can they convince you that you are good enough.
The tricky part is that you probably started liking them because you thought they were above your league. If you suddenly started feeling that you are on the same level, your feelings might fade. You might feel as if you might have miscalculated and actually your love interest is not so perfect after all. In my experience the relationship where one side is longing for the other to validate them and make them feel as if they are good enough actually do exist because of this unbalance. See, when you need the other person to validate you, it is because you believe that once they confirm you that you are good enough, you will finally feel this yourself. If this happens and your love interest indeed falls in love with you, you feel disappointed, because, well, your feelings about yourself have not changed. Instead, you start thinking that he might be the wrong one after all.