How can I be happy in Heaven?

Some people think that Jesus was Jewish, others speculate that his new religion was just an evolution of already existing ideas in Essene Judaism. However, in reality, Jesus had a far bigger influence on this philosophy: the silk road.

Joseph went into exile with Jesus to avoid Herod's purge - - - right from day one. The family needed to be out of Israel but somewhere that lots of Jews were, so that Joseph could get work as a carpenter. Also, due to Herod's spies and informers, they wanted never to stay in one place for very long. 

The answer was to join the silk road on a large party of caravans (wagons containing goods being taken from town to town for sale along the silk road).

The silk road was full of different cultures. The route would pass through Persia, Saudi Arabia, Afganistan and parts of northern Sindhu (now called Pakistan, but then a Buddhist /Hindu domain. 

Due to the interaction of the young Jesus with religious and philosophically inclined individuals along the way, Jesus became heavily influenced by the religious ideas stretching all the way along the route, from Egyptian and Greek ideas from the west, to Zoroasterian, Buddhist, Advaita and multiple Hindu ideas from the east. 

This shows in Jesus' philosophy written in the earliest of known Christian scrolls: The so called Thomas's Gospel (which only contains a selection of sayings, like notes jotted down from seminars or lectures). The Thomas Gospel clearly shares themes with the Bhagavad Gita. 

As you may know, Hindu religions often contains reincarnation mythology, and also ideas and notions pertaining to the evolution of the human soul. 

According to Hinduism, a being has to live many lives and under go many experiences before it attains perfection and becomes one with the Divine

There are nasty planets called asurya lokas including a Hell planet called Naraka loka and run by a devil character called Yama  (who tortures wicked souls as punishment), and there are multiple higher planets that humans can work towards, culminating in Satya loka  (literally planet for the truthful). 

It is possible that the Romans had concepts of Heaven and Hell or that Jesus brought aspects of Hinduism into Judaism, thus updating the idea of Sheol, more of a Hades type idea. 

So to answer your question, as Jesus is essentially borrowing from Hindus, the notion is that bad relatives would go on to other planets (lokas) to learn from their errors, thus slowing reaching the higher domain's. 

Jesus probably didn't believe in eternal torture. I think the Romans made that up to scare people into following the rules. 

Those in Heaven agree with God's Justice and are happy the people in hell are getting the kind of eternity they chose for themselves.  No one goes to hell without his own consent.  As Dante said when speaking of having concern for those in hell:

Here all pity or all piety must die.

Since heaven doesn't exist you don't have to worry about it.

Because you will "forget" the people who don't make it.