Why You Shouldn't Store Milk In The Refrigerator Door
There may not be any use crying over spilled milk, but spoiled milk is definitely worth a tear or two. Wasted milk is sad no matter which way you splatter it, but spoiled milk especially is so easily avoided you can't help but get a little sour at the thought of it souring.
If you want your milk to keep longer than it has been, you need to stop storing your milk in the refrigerator door. As you probably already know, dairy products don't do well in hot and humid conditions (as a matter of fact lots of foods don't do well in hot and humid conditions, but dairy especially), and your fridge is specially designed to be as cool and as dry as possible to ensure your food stays as fresh as possible for as long as possible, per EatingWell.
Your fridge door is opened and closed a couple times a day, and every time it's opened, the contents in the fridge door get exposed to some dramatic temperature changes as it swings out from the fridge and into your kitchen. This constant disturbance is not great for the integrity of your food and so you should not keep milk, or any other dairy products for that matter, in the fridge door if you can help it.
Milk's proper place in the fridge
It may be a little less convenient, but keeping your milk deeper in your fridge and as far away from the temperature-changing door as possible is your best bet. Milk is generally best kept at about 37 degrees Fahrenheit — right in the middle of a refrigerator's perfect temperature range. Food & Wine suggests that most fridges are coolest and the least disturbed toward the back on a lower shelf.
The "Today" show recommends storing your milk toward the back in the upper shelves of your fridge, as this is the area with the least amount of temperature fluctuation in most refrigerators. All in all, the general consensus for milk placement depends mostly on your fridge, where the air vents are, how much space there is, etc. but all agree that your milk and all your other dairy products need to be kept cool and consistent and decidedly not on the fridge door. No matter how convenient it may be, spoiled milk is no use to anyone.