Cycle of Hunger - Satiety - Hunger
Humans go back and forth from the state of Hunger to Satiety every single day.
Unknowingly, we think and talk more about food than anything else.
It is one of the decisions, that you might not be making rationally after all.
The sensation of hunger depends primarily on internal and external factors.
Internally, your stomach has contractions or pangs which in turn depends on your age, age inversely proportional to the gastro-intestinal muscle movements.
Also, two major hormones involved in this are - leptin and ghrelin. Desire to eat implies higher levels of leptin which in turn release Ghrelin, re-initiating sensation of hunger.
Metabolism rate is governed by your Hypothalamus. It tries to maintain optimum weight and fluctuates the metabolism rate accordingly. Thus a healthy hypothalamus maintains your body weight.
Then we have the external forces such as emotional state (say, tired, anxious, elated), stress level, social setting (say, a party, family get together, etc) or simply sight/thought/smell of food.
When it comes to food, internal and external factors are not necessarily dependent on each other.
The entire cycle is heavily influenced by our environment - upbringing, culture, routine; thus inducing learned and cognitive eating.
Remember the time while sitting in classroom, waiting for the lunch break, waiting for the clock to strike 1, you had learned and trained yourself to be hungry at 1, the official lunch time. It was not necessary that your body needed nutrients/sugar, etc. at that point of time.