Sweeten up phrase
To cause food or drinks to have a sweeter taste by adding sugar, etc.
After sweetening the sauce up, I tasted it again.
I used condensed milk to sweeten up the smoothie.
To make someone friendlier or happier.
She sweetened her son up with a new toy, but he got fed with it pretty soon.
I know that he was just trying to sweeten me up to borrow my car.
= sweeten (up) the kitty.
The verb "sweeten" must be conjugated according to its tense.
The origin of this phrase is not clear.