There are three reasons why Hell is eternal.
Reason 1: There is no other way.
Jesus asked His Father, in the garden before he was taken to be executed, if there was any other way to bring salvation to mankind. Three times, He asked the Father if there was any other way:
Then Jesus came with them to a place called Gethsemane, and said to the disciples,
“Sit here while I go and pray over there.”And He took with Him Peter and the two sons of Zebedee, and He began to be sorrowful and deeply distressed.Then He said to them,
“My soul is exceedingly sorrowful, even to death. Stay here and watch with Me.”He went a little farther and fell on His face, and prayed, saying,
“O My Father, if it is possible, let this cup pass from Me; nevertheless, not as I will, but as You will.”Then He came to the disciples and found them sleeping, and said to Peter,
“What! Could you not watch with Me one hour?Watch and pray, lest you enter into temptation. The spirit indeed is willing, but the flesh is weak.”Again, a second time, He went away and prayed, saying,
“O My Father, if this cup cannot pass away from Me unless I drink it, Your will be done.”And He came and found them asleep again, for their eyes were heavy.So He left them, went away again, and prayed the third time, saying the same words. - Matthew 26:36-44
http://www.biblegateway.com/passage/?search=Matthew26:36-44&version=NKJV
He asked that if there was any other way that the cup of responsibility be passed from Him, but since there was no other way, because the debt that sin generates against the demands of justice is infinite, and only someone with infinite worth could pay that debt.
Reason 2:
Love must be freely given.
If love can be compelled, if a woman just doesn't like a man, and he's been pursuing her and finds a way to give her a pill that now she likes him, if that were somehow ok, and not grotesque, well then maybe we could think that love could be compelled. But love cannot be compelled. Love must be freely given.
Reason 3:
The Bible emphatically tells us that those who reject God will live forever in separation from Him.
If the punishment for sin was finite, if it was not eternal separation from God, then surely God, in all His creativity, could come up with a payment for salvation, something other than the life of His only Son. For there is nothing in all of existence, more precious, more valuable, of greater worth, of limitless atonement, than the life of God the Son, yet God the Son became flesh, dwelt among us, then offered Himself on the cross, was separated from the Father (which is death), the Father poured out His wrath on the Son to pay for our sin.