Keep somebody sweet
Definition 1 of 1
Pro Tip 1/3Flattery usage
Often used when flattery is a tactic to get something from someone. 
I complimented his cooking to keep him sweet before asking for the recipe.
Pro Tip 2/3Informal context
Used in casual or personal situations, not suitable for formal writing. 
To keep her sweet, I agreed to go to the movies she wanted to see.
Pro Tip 3/3Manipulative hint
Can imply manipulation or a hidden agenda behind kind actions. 
He always brings gifts to keep them sweet, but he expects favors in return.