Because you’re not starting out with all of the information, and you are probably not starting out with the correct information either. When you idealize someone, you are setting up a picture in your mind of what you want them to be like. When they turn out not to be like what you wanted them to be like, you feel disappointed, and often hurt.
The other problem is when that other person has some of the characteristics of your idealization. Maybe he said something sweet to you, maybe you had some things in common, maybe he was especially attractive. So in your mind, you are thinking that if they have some of the characteristics, then they should have all of the characteristics. That is almost never going to be the case.
Because you don’t know them well enough in the beginning, you can’t know how they are in all of their characteristics. Even if they continue to have some of the good ones, if they end up having some that you particularly dislike, then it is hugely disappointing and you can tend to think that you mis-read all of the signs, or that your own judgement is impaired, or that you are a terrible person because you must have made them develop the unwanted characteristics. Most of that isn’t true, but because of all the fairy tales, and soap operas, and Hollywood bullshit, we are prone to believe that if you see some of the good stuff in the very beginning, then all of the other stuff that we want the other person to be like will be there too, but it rarely is.
Most people can never live up to anyone else’s idealized version of the perfect person. But when you meet someone that has some of the characteristics that you love, your mind often lets you believe that a person will have all of those characteristics.
That’s why it’s so much better not to have an idealized vision of what you will accept in a mate. It’s better to have “deal breakers” and then go from there. A deal breaker is something that is simply not acceptable to you, or in other cases, just something that you really don’t want to have. In some cases a deal breaker might be someone that smokes, in other cases, it might be (and should always be) someone who is abusive towards you. Most of the other things are somewhere in the middle, and not really horrible, just not your taste. Say you don’t like hairy backs, or you don’t like people who bray like donkeys when they laugh. Some things will be deal breakers for you, whereas they wouldn’t for someone else.
It’s also better to get to know people better before we plunge into serious relationships with potential mates. A lot of people simply have a physical attraction to someone immediately, and then they become intimate right away, without getting to know the person in a deeper way first. That is where a lot of the trouble starts.
I think we should tell Cinderella to hit the road. There is no such thing as Prince Charming. There is, however, a lot of potential lids for a lot of potential pots.