If you are in a situation that you need blood and I come to you with a pack of blood and tell you: Here you can use this, but be advised that there is one, and only one, virus of Ebola on it. Would you use it?
1 - Holiness - It must be absolute:
God planned and created all creation to be absolutely perfect, because He is absolutely perfect.
So it's not a case of how much you sin but IF you sin. Adan and Eve sinned only once and were expelled from Gods presence... Think about that!
In the case of us, Humans, we were created to reflect his perfect character, and that for the eternity. Eternity means every single second. A single sin was enough to ban Adam and Eve from the garden... That what means Holiness: Be perfect always.
2 - Eternity - It's for real
So when we sin, we ruin God's eternal plan. Doesn't matter how long you have lived in sin or how many sins you do, God plan IS (God didn't gave up on His desire for us) an eternity of Holiness and we ruined it when we sinned. Now, BECAUSE of His grace and love for us, God is bearing the loss of these years of Holiness that He planned for us and appealing to us to repent and return to Him. He went to the lenght to offer His very own son so we could can back.But Hid grace have a purpose: Makes us repent. If we refuse to repent, the punishment of the Law (which is Holy) is applied.
3 - Hell - It's eternal
That explains why Hell is eternal: Because if you refuse to repent and return to God, you are destroying God's eternal plan for YOUR LIFE, which leads to eternal damnation ..... You destroyed something eternal, you will be punished for eternity.
God rules, not a single leaf falls from the trees without Him notice it, not a single fact of history happens without His authorization, not a single act you commit , good or bad, goes unregistered....
" It is a fearful thing to fall into the hands of the living God." Hebrews 10:31