If you have read Lolita, what made you read it for the first time?
The book is famous or considered a classic.
I had read other Nabokov books/short stories first, and this was a natural progression.
I heard it was supposed to be dirty.
I saw a movie version and then wanted to read the book.
Someone I know urged me to read it.
It was required reading for school.
The subject matter interested me.
The subject matter interested me due to my personal experience.
Other, which I will explain in a comment below.