Hi spacenut, great question! I think the jury is still out as to whether foods themselves are addictive (in the same way, say, as drugs are). Some people have recently (and controversially) said that they are, and we should be treaeting certain types of obesity in the same way that we treat drug addicts.
In an extreme form, food addiction is more to do with an obsessive/compulsive behaviour of the individual, rather than the food itself. In these circumstances, people are unable to control the way they eat food or the way they think about it, and thoughts of food become very dominant in their lives.
Of course, that’s an extreme, and lots of people regularly say things like “I’m a chocoholic” or something similar, meaning that they really like or crave chocolate. When we eat foods that are high in fat or sugar, they tend to cause serotonin to be released in our brains. Serotonin is a neurotransmitter which is partly involved in making us happy, so in theory, you could become fixated on the need to get the feelings that eating chocolate makes you have, but it’s not clear whether that’s the same thing as addiction.
I think a lot of it is behavioural – snacks and sweets are seen as naughty treats; something that you can have to reward yourself, but shouldn’t have too often. In this way then, becoming ‘addicted’ to chocolate may actually be the result of other things going on in your life which you might feel the need to compensate for with something nice.