SB 2.5.9 purport:

It is usual for the devotees to become very enthusiastic whenever they are questioned concerning the Almighty Personality of Godhead. That is the sign of a pure devotee of the Lord. Such discourses on the transcendental activities of the Lord purify the atmosphere in which such discussions are held, and the devotees thus become enlivened while answering such questions. It is purifying both for the questioners and for one who answers the questions. The pure devotees are not only satisfied by knowing everything about the Lord, but are also eager to broadcast the information to others, for they want to see that the glories of the Lord are known to everyone. Thus the devotee feels satisfied when such an opportunity is offered to him. This is the basic principle of missionary activities.