You joked with your friends every Valentine's day at Hogwarts saying that you would give an Amortentia potion to random professors and try and get them fired or something.

..this year, your friends dared you to prank the new D.A.D.A professor, Tom Riddle, for this year's Valentine's day you did and succeeded with the love potion.

You were resting in your dorm room when your owl flew threw your window with a letter.. a love letter, from Tom Riddle.