Do you know why this is?

For example, since I’m vegetarian, I often find myself craving protein (recently it’s been hard-boiled eggs) because my body is lacking it. I’m also a big sucker for the power of suggestion. If I’m watching a movie and they’re eating a delicious-looking salad…I want salad. In the next scene they’re eating a scrumptious cupcake…I want a cupcake: two completely different things.

When you crave something do you realize it is due to a deficiency of some kind or is it completely random?
And what’s with sweet cravings? I mean our body can’t actually need that Snicker’s bar…