Each sport has a position that generates more attention and is often more fun for kids to play. However, the desire to play one particular position often interferes with a player's overall development. Only by playing a variety of positions can a player learn all the basic skills required for advanced levels of play.
For example, playing an offensive position improves speed and control. However, playing defense is a good way to improve backwards movement and understand the importance of positioning. Different positions emphasize different skills. Playing in the same position all the time doesn't give players a good chance of learning all the different skills that are required by all players.
More importantly, playing different positions teaches players the importance of teamwork and the need for all positions to contribute to a winning game. Only when a player respects the difficulty of a teammate's assignment can players come together in team-oriented play.