The idea of 'sweetness' is an idea that the Puritans promoted heavily. They thought that the Christian life should be one that is enjoyed to the fullest. They also thought that this was the highest purpose to human existence, as well. Remember that the main purpose of life was glorify and enjoy God forever. Sweet!
"Every thing that a man leans upon but God, will be a dart that will certainly pierce his heart through and through. He who leans only upon Christ, lives the highest, choicest, safest, and sweetest life."