Only God owns human beings ultimately. So to be enslaved to God is a homecoming to our true master. What makes this so good is that the one true master and owner in the universe gives gifts, not wages. Because earning wages depletes us and we hope that the wage will make up the depletion. But getting gifts depletes no one.
Wages imply that the master needs our work, and so has to recompense it.
Gifts imply that the master does not need our work and does not have to recompense anything. The one true Master in the universe has no needs. And that is why he never pays wages. To those who trust him, he only gives gifts. That is why his yoke is easy and his burden is light.