Old English swete "pleasing to the senses, mind or feelings," from Proto-Germanic *swotijaz (cf. Old Saxon swoti, Swedish söt, Danish sød, Middle Dutch soete, Dutch zoet, Old High German swuozi, German süß), from PIE root *swad- "sweet, pleasant" (Sanskrit svadus "sweet;" Greek hedys "sweet, pleasant, agreeable," hedone "pleasure;" Latin suavis "sweet," suadere "to advise," properly "to make something pleasant to").
To be sweet on someone is first recorded 1690s. Sweet-talk (v.) dates from 1935; earliest uses seem to refer to conversation between black and white in segregated U.S. Sweet sixteen first recorded 1767. Sweet dreams as a parting to one going to sleep is attested from 1898, short for sweet dreams to you, etc. Sweet and sour in cooking is from 1723 and not originally of oriental food.