Watching your daily intake of calories is a healthy idea. Obsessively counting each and every calorie that passes through your lips, being constantly hungry because you are not taking in enough food or setting severe eating rules for yourself can all indicate that a healthy behavior has become an unhealthy obsession.
The amount of calories you should take in on a daily basis depends on your age, gender and level of activity, although as a general rule, the average man should consume about 2,500 per day, and the average woman should take in 2,000 per day [source: NIH]. To figure out exactly how many calories you need to maintain your weight, or lose weight in a healthy way, you can try using the American Heart Association's fat and calorie counter. Taking in fewer calories than you see recommended there can indicate that you've taking things too far in your quest to lose weight.
Another facet of this type of obsession is placing yourself on a severe diet, such as one in which you eat only a single type of food (like pastes or gels), replacing full meals with a glass of water, or chewing gum in lieu of getting yourself a nutritious plate of food.