Here's what I read, and maybe it's wrong. Dogs love chocolate, so when they eat one they are going to want more and that's when it gets toxic. Not to worry if your dog eats one M&M, which a lot of people panic over, but if there are more M&Ms around after the first they will crave more.
My little guy chews everything, too, and often I'm also looking at a wrapper wondering what the heck was in it. I keep the chocolate higher than he can reach, and never leave it where he can. I like M&Ms, and if I'm eating some, I never turn my back on them or the dog.
