It's not, because God gave us the power to choose, which is essential to the ability to love. To choose God is heaven, to reject him is hell. Why? Because God is the source of all goodness, life, and happiness. If you turn away from him, you're turning away from these things too.
What kind of heaven would it be if God forced those who don't love him to spend eternity with him? Would that be loving? Would it be any less a torment for them than hell?
If you believe in a loving God who allows evil to exist in the world, you can't very well deny hell on the grounds God wouldn't permit it. If God allows bad things to happen to good people in this life, why shouldn't he allow bad things to happen to bad people in the next, especially if they choose it for themselves?
In deciding for or against God in this life, we're determining how our stories will end, what we'll finally be. It's not something which God forces upon us, but what we choose. As C. S. Lewis noted, the doors of hell are locked on the inside.