The Bible says that God is not willing that anyone should perish, but that we all be saved. He originally made Hell for the devil and his fallen angels. He loved us soooo much that he sent his only begotten Son to die in our place. Jesus said it would be worse for some than for others in the day of judgement, inferring various degrees of punishment.
Why is Hell eternal? Jesus said it was, for one. But here is the logic. Jesus(GOD on earth-Emanuel), is an infinte being so he suffered a finite amount of time for all our sins. If we reject his provision, then we being finite beings must suffer an infinite amount of time and still wont pay for our sins. Time began when God put the world in motion, and will cease after God makes the new heavens and new earth, we will then be in eternity, which is forever.
Hell is a horrible thing to contemplate, but Jesus warned us of it more than he spoke of heaven, and he DIED for us so we don't have to go. Now if you hear the good news that God offers salvation as a TOTALLY FREE gift, and you do not heed the pleading of the Holy Spirit to accept the gift, and you die in your sins, there is nothing that can save you.
We have all sinned and come short of the glory of God, how do you know, by the ten commandments. Have you ever told a lie, then youre a liar, have you ever stolen anything no matter how small? then you are a thief. Have you ever taken the Lord's name in vain? then you are a blasphemer, have you ever looked at a woman to lust after her? then you are an adulterer. So right there you are a lying, thieving, blaspheming, adulterer in God's eyes and he cannot let you into heaven. In James it says if we break one small part of the law (10 commandments) we are guilty of all. God cannot let anything unclean into heaven, it would be consumed by His Holy presence instantly. He offered His Son as a covering for our sins, His blood washes away all our sins, removes them as far as the east from the west, which is infinite. But we all have sinned, he paid for our sins, offerred it as a completely free gift, we only have to accept it. He only provided one way to come to him and that is through His Son. He gave us over 300 prophecies in the old testament about Jesus to prove Jesus is his Son, so we have no excuse.