There is no scientific proof that dreaming about someone means the person is thinking about you. Consider dreaming about a person you do not know, such as a celebrity; this person cannot possibly be thinking about you if he does not know you.
Dreams are derived from your subconscious mind. The person that you are dreaming about is part of a larger sequence of thoughts that can symbolize repressed emotions. Dreaming about a childhood friend can be an emotion tied to your past or a present concern, such as fear of aging. Therefore, it is possible to dream about a person without a corresponding connection from the physical world.