Why do I always crave milk? It's been like this for as long as I can remember. I usually drink 1 to 2 gallons every day. It's like I need it.

Do not. It is hard to define the cause of a craving. Two gallon of milk s too much to drink each day. The calcium content can be problematic. Also if it is "whole milk" the fat content would be too high. It would be wise to have an evaluation including lab tests of lipids and chemistry.
Better stop. Habituation is a label we use with people caught up in a pattern of behavior.Cow milk is designed for cows and if you are drinking this amount, some basic problems arise. The intake cuts down your craving for other foods and the cow milk protein can cause microscopic bleeding in your intestines.This eventually leads to enough blood loss to be a problem.You need to see your doc for testing.