I believe that gluttony is a sin for the following reason.
Gluttony is the consumption of more than you need, instead of giving your excess to those in need.
Suppose that you have two hamburgers, and you eat both of them despite the fact that you needed to eat only one. Suppose that before you ate both hamburgers, you were aware that your neighbor had nothing to eat and needed to eat.
Love for your neighbor would have prompted you to give your unneeded second hamburger to your neighbor.
Selfishness would have prompted you to eat both hamburgers when you needed to eat only one.
Satisfying your unnecessary selfish desire while allowing you neighbor to suffer needlessly is what makes gluttony a sin.
Gluttony comes in many forms, not just in the form of eating.
Spending money on luxuries that you don't need while your neighbor is in need of necessities is itself a form of gluttony.
Gluttony is the consumption of more than you need, instead of giving your excess to those in need.
Suppose that you have two hamburgers, and you eat both of them despite the fact that you needed to eat only one. Suppose that before you ate both hamburgers, you were aware that your neighbor had nothing to eat and needed to eat.
Love for your neighbor would have prompted you to give your unneeded second hamburger to your neighbor.
Selfishness would have prompted you to eat both hamburgers when you needed to eat only one.
Satisfying your unnecessary selfish desire while allowing you neighbor to suffer needlessly is what makes gluttony a sin.
Gluttony comes in many forms, not just in the form of eating.
Spending money on luxuries that you don't need while your neighbor is in need of necessities is itself a form of gluttony.
