Joey tells us early on that "only one man was ever [his] master" (1.3). That's one faithful pet.
Joey and Albert's relationship is like that of another famous boy/animal duo: Lassie and Timmy. Albert's a good kid, always does what his mama tells him to do, and he's devoted to the farm and to Joey. Joey is very possessive and protective of Albert, even calling him "My Albert" (2.5). Aw. They may not have a ton of time together, but their bond sure changes Albert's life.
In fact, Albert enlists in the military and becomes part of the veterinarian medics, hoping that one day, out of all the vet tents in all the world, Joey will walk into his. Even though Captain Nicholls suggests that Albert join the cavalry, he goes a more peaceful route. We guess he's a healer, not a fighter.
Albert and Joey's reunion toward the end of War Horse sure will pluck at those heartstrings. Joey recognizes the gentle tone of Albert's voice, but it's his unique whistle that seals the deal:
It was his owl whistle, the same low, stuttering whistle he had used to call me when we were walking out together back at home on the farm all those long years before. (17.27)
Like long-lost brothers, Albert and Joey reconnect instantly, and Albert tells Joey all about his life back home. He talks of Maisie Brown and his now-sober father, whom Albert has found it in his heart to forgive.
The hope of being reunited with Albert keeps Joey going throughout the war, and once they're reunited, Albert literally saves Joey's life. Consumed by tetanus, Joey is on the verge of death, but when the soldiers recognize the intense bond between Albert and Joey, they put all their resources into saving the horse's life. If it were any other horse, we're not sure they would have put the same level of energy into it. But Joey's a brother to Albert, and brothers stick together—especially in times of war.