How can we be happy in heaven if someone we love goes to hell?
The short answer
The question thinks of heaven "inside the box," as an earth-like place with clouds added. Heaven is about God — literally. God is infinite goodness and unlimited love, beyond anything we can now imagine.
Outside the box
Heaven is not just another earthly situation with clouds and cherubs added, so we can't think about it in those terms.
Heaven is for God's sake
Ask yourself whether you really believe heaven is about God. It is entirely about him. God is infinite goodness, goodness without limit, pure unlimited love — beyond anything we can now experience or imagine.