When it's cold and dark out
Short, wintry days can make us crave carbs like bread and pasta. People who suffer from Seasonal Affective Disorder (aka SAD, which is depression related to diminished sunlight-exposure) may be especially affected and crave carb-rich foods to help themselves feel better.

When we're accustomed to eating
We often crave foods with associations to happy times we've had in the past. When we're at the ballpark, for example, we might crave a hot dog with the works; if a summer-night trip to the ice-cream parlor was a family tradition, we may find ourselves craving an ice-cream cone when the weather gets warm.