Eating in response to stress or negative emotional states is well-documented in both humans and animals in experimental settings and has been shown to work by alleviating the unpleasant emotional experience. This type of eating behavior, called stress-induced or emotional eating, is linked to the development of obesity.
Standard approaches to companion animal obesity have failed to incorporate this concept. Not every animal given more food than they need will become overweight, which begs the question: Why does the animal that overeats do so? If it is to help alleviate stress or negative emotional states, then the standard obesity management approach of restricting food intake without alleviating the emotional distress may actually exacerbate the distress by removing one of the animal's coping mechanisms.
Moreover, because emotional eating is a coping mechanism, overeating may be a sign that an animal's psychological well-being is impaired.
Source: Franklin D. McMillan, 2013. Stress-induced and emotional eating in animals: A review of the experimental evidence and implications for companion animal obesity. JVEB online, September 2013. doi:10.1016/j.jveb.2012.11.001