A Vaisnava always feels himself incapable to repay his debt to his spiritual master therefore he works very hard in order to try and repay that debt.
There hasnt been much mail but one letter Srila Prabhupada replied today was from Catur Murti dasa, a devotee living in New Mayapur, France, who is having Prabhupadas books translated into Spanish and Portuguese. He has been preaching in Spain and he expressed his desire to try to please His Divine Grace by his efforts.
Srila Prabhupada was very happy to hear of his keen endeavor and surrender to his guru. He wrote back, "Please push on this movement in the Spanish and Portuguese languages with great enthusiasm and certainly Krishna will bless you for taking so much trouble to preach His glories. A Vaishnava always feels himself incapable to repay his debt to his spiritual master, therefore he works very hard in order to try and repay that debt. He knows that without the mercy of the spiritual master one cannot preach this Krishna consciousness and therefore he always tries to act in such a way that he may please his guru."
Reference:Transcendental Diary Volume 4 by Hari Sauri Dasa