Gluttony is considered a sin because gluttony is appeasing your body's desires and wantonly wasting resources. In addition it is very unhealthy! Gluttony can relate to obscene food habits and other such things.
Imagine a 500 pound fat guy eating food all day long. It's basically self-worship of your appetite and your body :)
When your needs are overwhelmed by your desires and you think enough is never enough you are in a destructive cycle that is harmful to yourself. Some call this sin, some call it stupidity, and some actually receive compensation from the government in the form disability checks because someone has in their professional opinion labeled gluttony as a mental disease treatable with $$$$..
Gluttony is condemned by God and does not harmonize with the divine quality of love. A glutton, being greedy, deprives others of their share of good things that Jehovah provides for all to enjoy. At a meal the gluttonous person may make it difficult for others to eat to satisfaction. By yielding to selfishness, he isolates himself from others. So if a person values the respect of others, he will avoid gluttony. Also, since Jehovah God created the human body with the proper capacity for food, any excess is bound to negatively affect us, as is recognized in medical circles today.
Further, Jesus knew the bad effects of overeating. In warning his disciples he said: “Pay attention to yourselves that your hearts never become weighed down with overeating and heavy drinking.” (Luke 21:34) Gluttony will affect the mind and heart making one lazy, drowsy and careless. In this state, he may be very negligent in his responsibilities. Therefore, it is the course of practical wisdom to avoid it. Earnest prayer may be required to overcome inclinations to overeat. (Luke 11:13; Gal. 5:22, 23) But avoiding gluttony is that important, and God will help those earnestly seeking his aid in this regard.
Do not be insatiable for any delicacy, do not be greedy for food, for over-eating leads to illness & excess leads to liver attacks. Many people have died from over-eating; control yourself, & so prolong your life. Ecclesiasticus.