Jessie asks via Twitter:
There are a few reasons that play into each other well. The first is psychological. Chances are good that as a youngster, when you got sick your mom, grandma, or some other nurturing figure would make you soup. Eventually, you would feel better. And, because the soup doesn’t taste like medicine, you have nothing but positive feelings around it. If that happens enough, then you are conditioned to enjoy soup, and it becomes a comfort food.
The next reason is that, when you get sick, you often become dehydrated. Fever, tummy troubles, or loss of appetite will reduce your fluid intake. When you run low on water, it makes it more difficult for the body to pass nutrients around. You’ll become weaker, and you won’t get better as quickly as you would properly hydrated. Soup, being more liquid than most foods, helps with the rehydration.
Soup is also often easy to eat and digest. Even with a sore throat or gastrointestinal problems, soup, depending on the kind, is often easy to swallow and easy to keep down. Even if you can’t eat a complex soup, a stock, vegetable or meat-based, will generally be happily accepted by your body. With such a combination of easy to eat, easy to digest, and nutritional value, it’s not surprising that your body enjoys it.
And, in the case of aggressively seasoned soups (especially processed soups), salt is sometimes what your body needs. I am not a big fan of canned soups, but even they can have their uses in the right circumstances. Sore throats respond well to high salt concentrations, so heavily salted foods can help that. Personally, I would recommend instead a salt gargle and some good soup instead of canned soup, but it is one of the reasons that it is so comforting.
Possibly the best reason that soup tastes so delicious and comforting when you are sick is because, often, someone serves it to you so you can just lie there and moan, coughing weakly. Or perhaps that’s just me.