Marketing: The process of communicating the value of a product or service to customers.
Marketing is the process of building a business relationship with potential customers. Have you ever established a lifelong friendship with someone you had contact with only once? Probably not. Generally friendships (and all relationships for that matter) grow as a result of frequent contact over time. Even when the potential to form a great friendship is there at the first encounter, it is unlikely it will grow without nurturing.
Seth Godin in his book Permission Marketing uses an analogy of seeds and water to demonstrate the importance of assuring adequate frequency in your promotional campaigns. If you were given 100 seeds with enough water to water each seed once, would you plant all 100 seeds and water each one once? Or would you be more successful if you planted 25 seeds and used all of the water on those 25 seeds?