You would assume that everyone knows what they want in life, but the thing is, we don't. Ideas and reality are two different things. You can think up the most perfect recipe in our head, but when you try it for real, it can end up being the complete opposite of what you imagined. The same goes for people.
We believe that we know what things we want and what kind of people we like to surround ourselves with. We know what characteristics we like in people, what we admire, and what we despise. With these ideas, we end up making some sort of criteria in our heads, similar to a check list. And when someone doesn't fit that criteria, we tend to write them off. Well, nothing can happen with this person because they aren't this, or they don't have this, etc.
Ever think to look outside that box? To give someone different a chance? Give them a chance to prove you wrong? I'm guilty of not giving those people chances. However, now speaking from experience, what you least thought you wanted can end up being exactly what you were looking for. Open your eyes and heart, and give your brain a rest every once in a while. Just because something makes sense in your head, doesn't mean it will appeal to your heart, and vice versa.
Give people the opportunity to surprise you. You never know what they could have in store.