Definition 1 of 1
Interestingly enough, it is also used when someone has done you great harm, intentionally or unintentionally - at such time this phrase can be used with a sarcastic tone to tell the offender that you're not amused.
Jaya : *Squeels with delight* Aaaaeeeeeee!!! Soooo sweet of you ya!
Hormese : Don't worry ra, when your girlfriend asked me where you were last night, I told her you were NOT out with your other girlfriend.
Josekutty : *mind blows a fuse, then says sarcastically* Soooo sweet of you ya!