Naw, I probably can't explain it well enough as to "why".I know myself, now that Pascha is over and we have feasted, I can "rationalize" eating again with the "it's an indulgence but what the heck, it is a feast and I am worth it".Can you explain why indulgence can be a sin? Is indulgence a sin, is it always a sin?
All is true in one sense, and ti's an indulgence as well. Continuing down that path dulls all kinds of things.
The actions may not be good for my spiritual life as well as the physical one, whether they be gluttony, lust, anger, and other sins, they all involve a justification of one's self, also known as Pride.
You are worthy in the Eye of God, as He made you and loves you. You are not worthy of the sins you desire for yourself because of His Love.
The word "indulgence" means there is an activity or state of being that one is IN, or involved in.
One indulges in something, like play or eating or some activity, no?
As moderation is a virtue or goal for us to practice, indulgence could be considered a violation of such.
One can indulgence in something that is already a sin.
Also one can indulge in the love and / or joy of Christ and finding a sin in such would be difficulty, no?
If you have a specific thing in mind, put it out on the table. Hope this helps a little.