I think it's all about attraction and having some things in common. When you've spent a considerable amount of time with someone and you discovered that such someone is not as exciting as you thought he/she would be, you'll easily lose interest in such a person and look for someone that provide the sort of excitement that you seek. Also, most people go after those that seem not to have interest in them because they like the chase or want to prove to themselves that they can get such a person.