An eternity spent apart from God (idea)

for now the thought
Both of lost happiness and lasting pain
Torments him

According to Milton, Hell is truly Hell because of the lost glory of Heaven. Imagine, even if you don't believe in God or an afterlife, that after your death, you were given a glimpse of perfection: a place without sadness, need, hunger, illness, pain, death, murder, lust, greed, rape, or sin. Now imagine having that taken from you. Your perfect world does exist, but you may never have it; for all eternity you will spend your days in a place far from God, far from perfection, even if not in a burning lake. Now, you might argue that peace and contentment is in the mind alone, but what if it were revealed to you that the only place you could attain such a state of mind was in this perfect world? Hell is eternal torture, the vast expanse of time and space and mind apart from contentment, apart from everlasting joy, apart from neverending love.

Regions of sorrow, doleful shades, where peace 
And rest can never dwell, hope never comes
That comes to all