Fifteen years ago, it was a common knowledge at Hogwarts that the Ravenclaw flower had a crush on the Slytherin half-blood student.
Fifteen years later, everyone in Hogwarts knew that the gentle, intellectual, mature and beautiful Astronomy Professor liked the mean, cold, greasy, and extremely unfair Potions Professor. Professor Snape also liked her, and everyone in Hogwarts knew it tacitly.
Purpose: Give the professor a good ending