Before you ask, "Why is Jake posting on such depressing subjects?", I must say two things. One, if you your book to be realistic, it has to have some sort of a depressing event. :) Second, because this is rather a continuation of my previous post, rather than a stand-alone--because disaster without suffering isn't a disaster at all. (Yes, that quote was completely original from me.)
Why should your Protagonist suffer? Because the reader can relate to suffering. And when the reader can relate to the character, that means you've done something right.
There are two kinds of suffering--that's easy enough to see. External (physical) and internal (mental).
This kind of suffering is the less effective of the two--you can only beat your Protagonist up so many times. But there are several ways to make this effective.
First; don't describe it in detail. Do you think a reader wants to read two pages about your Protagonist's bloody wound and how much it hurts? I, for one, would probably shut the book if that came up, or just grimace and hope that the rest of the book is better.
Second; don't be repetitive. Don't make your Protagonist be beat up every day. It gets boring.
Third; use it sparingly. Too much physical suffering will turn your reader off, and too little will give the Protagonist a feeling of invulnerability.
Fourth, and most important; it has to make sense. If your Protagonist has to get beat up, then make him get beaten up because of something he did (like inadvertently getting on the guy's bad side), or because of one of his qualities (because he didn't do something the guy told him to because it was wrong). The suffering event should be indirectly (at least) related to the plot and something it caused.
The subject of mental suffering is much more effective than physical suffering. Pretty much everyone experiences some degree of mental suffering (now I sound like a psychologist!), and if you pull the suffering off in a way that the reader can relate to, you create character development.
I must admit that I am not a very authoritative writer on the subject of mental suffering. I don't know how much I can show you about 'pulling the suffering off', but I'll do my best.
But first, let's define mental suffering. It's basically suffering from the mind, right? But what does that mean? It's when the Protagonist is struck with a mental blow; loss of a loved one, a thing important to the plot, hopelessness, etc.
Now, what are some things one can do to improve mental suffering?
First, don't have it dominate the plot for it's duration. Other stuff can happen too. Just like physical suffering (although this one DOES require more description), don't put too much description into the torment.
Second; use the Protagonist's thoughts to convey the suffering that s/he's feeling within, but use his/her actions, too. Show, don't tell, remember?
Third; It doesn't nescisarily have to be something huge, like the loss of a loved one. Something as simple as a cooling friendship can fill the Protagonist's thoughts at night.
Fourth; have the implications of that suffering stretch farther into the plot. For instance--the death of a Protagonist's mother may make him/her move to a different town, where s/he meets a mysterious stranger and gets embroiled in a... Well, you get the point. :)
As for the connections of Suffering with Disaster, I'll say again what I said above; disaster without suffering isn't disaster at all. Disaster and suffering alike have implications; disaster makes suffering (and more plot, probably), suffering makes the plot go in a different direction, which steadily causes an unstoppable chain of events which lead up to a smashing ending, each disaster and suffering driving it forward like a train on paper into the final chapter.
Oh, and as for that idea about writing a story with the sole subject of suffering--probably not a good idea. :) Writing intense suffering oftentimes means terrible misfortune, disasters, and a slew of characters to kill off. And that's depressing to read and depressing to write. :)
Seriously; I get the blues every time I kill off a character, good or bad. It makes me go and read a happier book. :)