Seeing a cue from a past reward triggers a dopamine release in the brain even if you are not paying attention to it.

Researchers at Johns Hopkins University found that our brains are biased by past pleasures. When you spot something you once enjoyed, your brain gets a sudden splash of dopamine. This happens even if you are not looking for a reward or even consciously noticing the object. It is like an automatic radar for pleasure, which explains why a dieter cannot stop thinking about cake the moment they walk past a bakery.