There is no duty, O Jajali, that is not done by some motive (of happiness). Duty, however, is very subtile. Duties have been ordained in the Vedas for both Brahma and heaven.
अकारणो हि नैवास्ति धर्मः सूक्ष्मो हि जाजले। भूतभव्यार्थमेवेह धर्मप्रवचनं कृतम्॥