From these verses it is clear to me that hell is eternal, I had to deny it too until I started reading the bible and realized how merciful and just God is. According to the bible many people will go to hell for eternity, BUT people will be judged according to their works. God knows how much people can handle so he won't put them in more pain than they can handle. A lot of people think hell is the same as being burned alive here on earth for eternity but that would be us figuring out what we can't fully understand with our earthly minds which is a common but fatal error.
To answer the question of why people are sent to hell. God's requirement for heaven is believing so if this is the way that God decides people go to heaven than we know it is the most just way of deciding for the individual who doesn't believe. We can't know how it is just as we can't know everything that plays out in their lives, but apparently it is and who are we to question God and his infinite wisdom on everything and everybody. So if you've had a good friend that you love and think is in hell, know that God is merciful and just. No, hell won't be happy like heaven but I know it isn't the same as being burned alive forever, not reading the Bible and using our own understandings causes this awful thought.
Anyways, on the positive side of things all of us who know Christ died for our sins get to spend eternity in heaven.
Revelation 21:8
But as for the cowardly, the faithless, the detestable, as for murderers, the sexually immoral, sorcerers, idolaters, and all liars, their portion will be in the lake that burns with fire and sulfur, which is the second death.”
Revelation 20:15
And if anyone's name was not found written in the book of life, he was thrown into the lake of fire.
Mark 9:43-48
And if your hand causes you to sin, cut it off. It is better for you to enter life crippled than with two hands to go to hell, to the unquenchable fire. And if your foot causes you to sin, cut it off. It is better for you to enter life lame than with two feet to be thrown into hell. And if your eye causes you to sin, tear it out. It is better for you to enter the kingdom of God with one eye than with two eyes to be thrown into hell, ‘where their worm does not die and the fire is not quenched.’
Revelation 20:14
Then Death and Hades were thrown into the lake of fire. This is the second death, the lake of fire.
Matthew 13:50
And throw them into the fiery furnace. In that place there will be weeping and gnashing of teeth.
2 Thessalonians 1:9
They will suffer the punishment of eternal destruction, away from the presence of the Lord and from the glory of his might,
Revelation 14:11
And the smoke of their torment goes up forever and ever, and they have no rest, day or night, these worshipers of the beast and its image, and whoever receives the mark of its name.”
Revelation 20:10
And the devil who had deceived them was thrown into the lake of fire and sulfur where the beast and the false prophet were, and they will be tormented day and night forever and ever.
Daniel 12:2
And many of those who sleep in the dust of the earth shall awake, some to everlasting life, and some to shame and everlasting contempt.
Luke 16:23
And in Hades, being in torment, he lifted up his eyes and saw Abraham far off and Lazarus at his side.
Revelation 19:20
And the beast was captured, and with it the false prophet who in its presence had done the signs by which he deceived those who had received the mark of the beast and those who worshiped its image. These two were thrown alive into the lake of fire that burns with sulfur.
Matthew 13:41-42
The Son of Man will send his angels, and they will gather out of his kingdom all causes of sin and all law-breakers, and throw them into the fiery furnace. In that place there will be weeping and gnashing of teeth.
Matthew 7:13
“Enter by the narrow gate. For the gate is wide and the way is easy that leads to destruction, and those who enter by it are many.
Revelation 20:13
And the sea gave up the dead who were in it, Death and Hades gave up the dead who were in them, and they were judged, each one of them, according to what they had done.
Ecclesiastes 9:10
Whatever your hand finds to do, do it with your might, for there is no work or thought or knowledge or wisdom in Sheol, to which you are going.
Mark 9:48-49
‘where their worm does not die and the fire is not quenched.’ For everyone will be salted with fire.
Matthew 12:36-41
I tell you, on the day of judgment people will give account for every careless word they speak, for by your words you will be justified, and by your words you will be condemned.”
Revelation 20:12-13
And I saw the dead, great and small, standing before the throne, and books were opened. Then another book was opened, which is the book of life. And the dead were judged by what was written in the books, according to what they had done. And the sea gave up the dead who were in it, Death and Hades gave up the dead who were in them, and they were judged, each one of them, according to what they had done.