"All men must die" signifies the inevitability of death, and "All men must serve" signifies that we all need to perform our roles in the world.
In this case, why is "Valar Dohaeris" the traditional answer? How does "Valar Dohaeris" fit as the response to "Valar Morghulis"?
Is there an implicit ".. but first.." between the sayings? Or is it purely tradition at this point with no real explanation?