Where do ya think your gonna put two reasons that big?
Bend over and I'll show you.
1. A "Real" Family
The Griswolds are about as real as we can hope to get in a Christmas movie. Dad is going to accomplish what he wants to accomplish, even if the whole family is against him. Mom is NOT stressed. So what if her smoking habit is back? The kids are happy that they are not at school but is being home for that long all it's cracked up to be? Let's be honest, you love your family to death but sometimes they make you want to play in traffic without reflective gear. This movie perfectly captures that. Sometimes you do things that you don't want to do for your family, and you smile even when it is hard. Clark doesn't have to let anyone stay with them but it's Christmas and he does more than he has to do. He even makes Rocky and Ruby Sue believe in "Santy Claus". The Griswolds are a breath of fresh air when compared to other "average" families that live perfect lives and find the lesson in every wrong. Sometimes your Cousin Eddie kidnaps your boss and sometimes you don't want Grandma Nora to give you a whole quarter but that's life. To an extent any way.
2. Social Commentary
Christmas Vacation does contain quite a bit of social commentary. I'm not saying it's Freudian but it is intelligent. We see a man, Clark Griswold, who just wants his family to "have the hap, hap, happiest Christmas since Bing Crosby tap-danced with Danny f***ing Kay." I think we can all identify with that to a certain extent. We want Christmas to be perfect. We want everyone to be happy and we want our Christmas bonus. We want these things so bad that we lose ourselves and forget what Christmas is all about. I don't think it was intended as anything other than a joke but as Aunt Bethany says, Grace died thirty years ago. It's a funny joke and maybe that's all it is but maybe it's a little bit more. Christmas Vacation does truly, even if indirectly show us that we need to remember that we celebrate Christmas because Jesus Christ was given as the ultimate gift for all of time.
Also, Die Hard IS a Christmas movie...