It's also just a fantastic movie. It's kind of like "A Christmas Carol", but instead of Ebenezer Scrooge being saved, it's the guy who's standing up to him. It's warm, it's funny, and it asks a lot of big questions. And it doesn't appeal to a single viewer. And it's not too heavy, either - Citizen Kane might be better, but it's not really a film you can get a 6 year old to watch.
There's very few movies which are so critically acclaimed (top 100), which can appeal to adults and kids alike. It's subject is both completely commonplace, and the most important thing there is.
Also, how many good Christmas movies are there? It's got to be in the top 5 Christmas films, at least.