Why was Barker offended when MacDonald mentioned Douglas’s wife? What did Barker tell him and Holmes about Douglas’s reaction towards his wife and Barker whenever they communicated?

From the text:

I may tell you that poor Douglas had just one fault in the world, and that was his jealousy. He was fond of me—no man could be fonder of a friend. And he was devoted to his wife. He loved me to come here, and was forever sending for me. And yet if his wife and I talked together
or there seemed any sympathy between us, a kind of wave of jealousy would pass over him, and he would be off the handle and saying the wildest things in a moment.