An afterlife would be terrifying for me

[Belacqua]

Can you give some examples of such complex ideas of happiness in afterlife in Christianity? This is interesting.

It's a fascinating topic!

There are a lot of variations, of course. One thing you can count on with Christians is that they'll disagree with other Christians. 

It's probably simplest to start with Dante, whose descriptions are in nearly all cases (allowing for poetic license) standard Catholic views. 

For him, God is the form of the Good, as in Plato. The more people have access to the Good, the happier they are. In the material world, in life, our access to the Good is limited and divided. What goodness we perceive in the world is an emanation from God, and shows some tiny percentage of what goodness is. (If we can speak of a percentage of something infinite.) So moving toward what we perceive as good both makes us happy and also brings us nearer to God. In the material world, goodness is divided into different aspects, so we perceive different good things, but these have their source in the one Good, and in heaven are united into One. 

For many Neoplatonic Christians, like Eriugena (though perhaps not Dante) the Fall of Man didn't occur because of disobedience, but because our perceptions of the world shattered into division -- we lost the view of the One, which is the Good, which we will have in heaven.

We are likely to be mistaken in what we perceive as good while it is divided in this world, and this is where Dante follows the Nicomachean Ethics almost completely. When we love the good properly, we have good results. When our love is misguided, this is called sin. Sin is based on love for something which we think is good but isn't. 

Heaven is direct unimpeded access to the form of the Good. It is, like the Platonic forms themselves, outside of matter, outside of space and time. There is no change, playing around, having parties etc., which sounds boring to us because we can't conceive of being without time. Those things demand division, though, and a partial view of the Good which is incompatible with the full unity of the One.

I'm not saying that anyone should believe this. Only that the standard theological view of heaven in Christianity is not simply a pretty picture of an improved earthly life.