All behaviors have a biochemical basis...
To give you an example let's look at schizophrenia, for a 100 years' schizophrenia was seen as a behavioral disorder. Today we know it's due to a lack of dopamine, and or a defect in glucose transport across the brain.
These are biochemical problems, that ultimately manifest themselves as a behavioral disorder.
So, what are the biochemical underpinnings of obesity?
Quick bit of science...
Leptin is a satiety hormone that signals when you are feeling full. It's released from your fat cells, and sends a signal to your brain that you have enough energy on board to engage in normal metabolic break down processes, so that you can burn energy at a normal rate.
Insulin is another hormone that does the same thing, when it senses that you are full it tells your brain, you've got enough energy, so let's burn some fat... this is how eating raises your metabolism, (and dieting can slow it down).
Problems arise when our insulin levels get too...