Be sweet on somebody
Definition 1 of 1
Pro Tip 1/3Casual Romance
Used to describe light-hearted romantic feelings more than deep love. 
I think he's been sweet on her, always smiling when she's around.
Pro Tip 2/3Not for Heartbreak
It's not used for situations of serious heartbreak or long-term relationships. 
She's been sweet on him, but not heartbroken since he left.
Pro Tip 3/3Youthful Tone
Often suggests a younger, more innocent type of affection. 
Ever since they were kids, he's always been sweet on her.