Many rejections are personal. If the person who turned down someone who asked them out is already in a serious relationship, is of a different sexual orientation than you believed, is a member of a religious order, then by all means don't take it personally. But rejections, especially of someone you've gotten to know a bit, and who has gotten to know you, can be quite personal. A rejection in most cases is in effect telling the person that they're not good enough to date. Yes, they may not be the person's "type," but "type" usually means "person to whom I'm attracted." At the very least, you're being rejected on the basis of your looks, which while superficial, are still a major part of a person's identity, and form the outer "shell" of how they present to the world. So, you are being rejected on the most public-facing aspects of your persona.

Moreover, there may be some instances in which it's useful to take rejection personally, as it may be a catalyst for change (where such change is possible). It may spur someone to get in shape, dress better, rethink their facial hair, take up additional hobbies, or whatever. Of course, in those cases in which someone's rejected on the basis of their bone structure/facial appearance or height, there's little to be done.