It is impossible to say that you are a true fan of anything, while not possessing an understanding of its roots. There are some films that are cornerstones of filmdom, and if you haven't seen them, you can't truly say that you are a lover of films. Casablanca is one of those films.
I personally waited a long time to see this film. And after I saw it, I wished I could un-watch every film I ever saw and see this one first. I felt cheated, having grown up with films influenced by this one film. Casablanca is a fundamental upon which one builds an understanding of film. So many films with themes of love, virtue and ethical dilemmas seem to draw from this one film great.
For a film which didn't hold any high expectations, Casablanca has got it all: a clever screenplay adapted from an unproduced play, great direction and production, brilliant acting, good chemistry and depth of characters. It received 3 Oscars and was nominated for 5 more, and is one of the most critically acclaimed and monumentally celebrated film works of all time. But then, you could find all that out in a quick visit to imdb or wikipedia.
Casablanca is one of the greatest films of the 20th century, and a must-see for anyone who aspires to call themself a film lover. There's just no nice way to put it: anyone who fancies him/herself a film buff, and has not seen this film, is fooling themselves. It's like claiming to be a music fanatic and not knowing Dylan.