you needed to return a book to the library, so you put the book in your bag and left the main hall, off to the library,

you opened the door to the library and went to the service desk,waiting for someone to show, until your eyes fell on a particular guy, sitting in the library, reading. The slytherin quidditch teams seeker, regulus. You went to their games a lot so you easily recognized him,

you had a crush on him for a while, so when you saw him you just kept staring.