What people without strong values don't understand, especially when it comes to duty, is that those motivated by their duty get immense internal satisfaction from knowing that they performed their duty.  This is often asked when looking over the grave of someone who gave their life in the performance of their duty and is asked "what did they get out of their act?"  This could be a soldier, a police officer or another first responder, but there are others too.  While the fallen's act may have saved others or averted disaster, those who sacrificed often get called heroes.  But these select few do not view themselves as heroes as that is a label applied by others, these fallen are content in the knowledge that they lived their duty, did not shirk it, and did not cower, regardless of the ultimate consequences.  And that knowledge makes all the difference to these committed and dedicated few.  This is what duty is.

Duty is the essence of manhood.  George S. Patton

Do your duty and a little more and the future will take care of itself.  Andrew Carnegie

Never mind your happiness; do your duty.  Will Durant

Let men of all ranks whether they are successful, or unsuccessful, whether they triumph or not; let them do their duty, and rest satisfied.  Plato