By now it's just pure instinct, predatory reaction to someone entering his territory. England was his and bits of France of course, mostly the bits where absinthe was still made close to it's original recipe. But all in all, most of those whom slithered and writhed in dark places left it alone. England was in general too -polite- for the other denizens of hell, which was fine for him.
Before England it had been a few cities in the Middle-east, before that Greece. Of course then it was harder to protect what was his, to scare imps and lesser demons off his territory, and in a few memorable occurrences, demons of power slightly greater than himself. Of course in those cases he'd never come right out and done anything, but he hadn't lost that quicksilver tongue of his on the way out of Eden.
It's harder when a mortal works their way in, and ultimately rare. It is also harder to work out, much harder, for while a demon working it's way close is of no consequence, Aziraphale would notice his latest "Dear" missing. Besides he loves humans (or something like it) in all their horrible contradictory nature, he doesn't actively hurt them, never really did. But the instincts are still so strong, desire, anger, jealousy. Their lives are so short, and if he really cared to be selfless for once, he'd let it lay. Let Aziraphale have that fleeting instance of human companionship, wait it out on the sidelines till his angel was mourning the loss of one more brilliant light dashed out in the face of their immortality.
But of course he much prefers to be selfish, it's easier to find someone, something, for that mortal to leave Aziraphale over. Crowley is ace at tempting, the racial memory of humankind can attest to that, and really you can't fault him. It's just in his nature to protect what's his after all.
His territory, falling wherever Aziraphale went, and he would protect it with vicious self-centered joy.