If you're sweet for someone that just means you like them
to be sweet for you= to like you
*Jenna and Michelle are standing and talking. Jake walks up
Jake: Hey, Jenna. Do you have a ride home?
Jenna: I was going to get a ride with Michelle, but her car is kind of cramped. Do you mind?
Jake: Not at all. Come on.
Jenna: Later, Michelle!
*They walk away
Michelle: They are so sweet for each other.
Girl A: He is so sweet for you.
Girl B: What?
Girl A: He really likes you.
Boy A: I heard that girl is sweet for you.
Boy B: She what?
Boy A: They said she likes you and thinks you're cute.