If a person was very concerned about someone then he might see him in a dream. For example: A person might dream of his sick relative if he was worried about his health.
A person can dream of the one he likes or the one he had a crush on. This usually happens at early stages of a relationship (See signs someone likes you).
Sometimes the person seen in a dream is a symbol used by the subconscious mind to refer to a totally different person. Because sometimes the subconscious mind find's it uncomfortable to refer to someone directly symbols might of used (see dream interpretation).
If a person is thinking too much about another person for any reason then he might see him in a dream. Because dreams usually represent what's going on in the mind the things people are concerned about the most usually appear in their dreams.
Sometimes the subconscious mind uses important people to send messages to dreamers directly. If the subconscious mind wants to motivate someone to do something then it might pick up someone this person cares about then send the message through him in a dream (See why we have dreams).
The subconscious mind can send us a dream about a person in order to motivate us to peruse him. This happens when the subconscious mind believes that this person is very important to us.
Many scientists believe that dreams are all about encoding events that already happened into long term memory. If an important event happened concerning a person then he might appear in our dreams because that's the way our minds organize events into memories (See why we dream in symbols).
Sometimes dreams have the purpose of charging us with certain emotions. The subconscious mind can present a certain person in a dream in a certain setup to charge us with some emotions towards him whether those emotions are good or bad.
Sometimes the subconscious mind uses people to create a certain plot in the dream to send us certain messages. By using the right people the plot can be more convincing and the dream can achieve its goal of motivating us to do something.