Cheever’s narratives rarely end ‘happily ever after’. Why might a writer choose to make their characters suffer?

I had a debate a while ago with one of the students I'm living with, about where realism's (and by extension, suffering's) place was in fiction. He was of the position that fiction should be about escapism and positivity, and that too much negativity was needless. I tried to reason that sometimes a story needed to involve suffering for the sake of realism, to which he replied along the lines of "we don't need that kind of realism in fiction, that's what reality's for."
I relinquished the argument after that, since I could not adequately retort, but after thinking about it the next day, I realised the flaw in that line of reasoning.

Reality is for being in pain is it?

Reality is for losing a relationship with ones father due to his alcoholism (as in Cheever's Reunion)?

Reality is for being consumed by the horrors of the cosmos despite trying to use their power for good (e.g. Michael Moorcock's Elric of Melniboné series of stories)?

I have constantly held that stories are a method by which we understand ourselves and the world around us. This can be seen from the mythology of ancient civilisations, which created Gods to explain the forces of nature and reflect our own flawed, temperamental image, to the TV dramas of today which create parallel worlds where people mirroring ourselves are brought to tears on a weekly basis.

We cannot go out and experience every piece of suffering reality has to offer, neither can we turn our heads from all the suffering and live in flights of fancy.
So what do we do? We use the power of fiction. We create characters who experience agony, and by reading, we follow their path down the road of pain, embracing the feeling of it, and hopefully we come to rationalise it.

Fiction can simply be a form of escape, but it can also help us face the cage.