Your dreams are a representation of everything that your subconscious mind has thought about throughout the day or the things that you've done. This means that when you dream it combines these things into something that might be illogical or might make complete sense. The reason you keep having dreams about the same person is likely because you either spend time with them or think about them often whether it is consciously or unconsciously. Then when you sleep your subconscious mind takes over and continues to think about all of those things. That same person then keeps getting put into the dream.