This is apparently one of my most noticeable quirks, and it just doesn't seem strange at all to me. From when I was a kid, all the way to today, I've always eaten one thing at a time. I can't really explain why this makes more sense to me than jumping around from item to item like normal people do. It's just a natural part of how I eat, and there's no particular "reason" for it. Usually, there is some string of logic to the order... for example, I like green beans hot or cold, so I'll usually eat them last, giving priority to foods that I'd rather eat while they're still warm. When eating steak fries, I'll usually eat them after the main course, but when eating scrawny fast food fries, I'll usually eat them first because they're thin and likely to get cold.
As far as I understand it, people jump from food to food because they like the mixing of the flavors. If something is prepared with multiple components and presented as a single dish, I have no problem with eating it as a one thing, but if it's presented as two separate foods, I eat them sequentially. Cooking is an art form, I do believe that, and the mixing of flavors can result in some amazing things... but in my eyes, if it's not prepared that way, it's a separate food, each to be eaten in turn. I agree that there are some foods that "go together," but that may be simply because I'm used to seeing them served together.
People act so surprised when they see how I eat. I just don't see how they change gears from "I want to eat chicken" to "I want to eat mashed potatoes" from second to second. To me, it's more like, that chicken deserves its own time to be enjoyed. For as wildly different as people can be, it amazes me that this manner of eating is so standard. Apparently this is just so bizarre that it's not shrugged off as "different," it's something that must be commented on and puzzled over.