Now, that is really simple to answer.
You see, we who are Saved, belong to the living God, Yeshua the Messiah. As His property, we are obliged to do His will, and His will is quite clearly stated in the end of the Gospel of Mark, for He has told us to:
Preach the Gospel to every creature.
So, in answer to your question, we are simply doing what our Lord tells us to do, much as you do what your lord instructs you to do.
Alas
I find it peculiar that many who identify themselves as Christian, excuse much of their obnoxious and boorish behavior by stating that they're "doing what their Lord tells them to do", but they seem to conveniently forget that their Lord commands humility, forgiveness, compassion, peace, and love for their fellow man.