This is one of the important doctrine of Dwaita Vedanta. The souls are always servants of Sri Hari whether before liberation or after liberation as the case may be. Even the Mukhtas in Moksha are only most obedient servants of Lord Sri Hari and nothing can be demanded as of right. The Lord grants them happiness according to their Swabhaava in the Moksha and such aananda is also due to his anugrahaa only.