Why did Abe leave the island, and why did he refuse to return?

From the text:

He said he wouldn’t be able to live with himself if he sat out the war while his people were being hunted and killed. Said it was his duty. I suppose duty meant more to him than I did. Anyhow, I waited. I waited and worried through that whole
bloody war, thinking every letter that came was a death notice. Then, when the war was finally over, he said he couldn’t possibly come back. Said he’d go stark raving. Said he’d learned how to defend himself in the army and he damn well didn’t need a nanny like the Bird to look after him anymore. He was going to America to make a home for us, and then he’d send for me. So I waited more. I waited so long that if I’d actually gone to be with him I would’ve been forty years old. By then he’d taken up with some commoner. And that, as they say, was that.”