Don't think you're some kind of slob if you have coffee stains on your cup. If, for example, you are fishing on a pond and got so carried away that you forgot about your strong coffee, then even after such a period of time, stains may already remain. All this is due to oils and tannins.
But the problem is that on stainless steel these stains stick most firmly. If you do not wash these stains for a long time, you may find that the coffee begins to taste bitter.