Srila Bhaktisiddhanta Sarasvati Thakura mentions that if one maintains within his heart the desire to enjoy the result of good work, or, being embarrassed by the material world, the desire to get out of material entanglement, one will never be able to attain the transcendental mellows of devotional service. In other words, one must not desire material profit when rendering devotional service. Even if one follows all the sixty-four regulative principles, he cannot attain pure devotional service with a contaminated heart.