People suffer because that's what we do. You'll never get a definitive answer to this question. There isn't always an answer to every question. Why do people insist that there must be? Can we not accept that some things will never be satisfactorily answered?
And if you do stumble across an answer that seems to work, as soon as you tell someone, they will almost certainly dispute it, because they have also been muddling along with their own answers, and I'll bet a bollock to a bottle of beer, none of them are the same as yours.
So in fact, asking this question, with the expectation of cracking it, will only result in lots of people disagreeing over each other's 'work in progress' models and paradigms, creating more suffering and misunderstanding in the World.
Often people need to be trained out of pointlessly inflicting suffering on themselves. All those little obstacles and behaviour patterns that aren't major enough to be classed as aberrant, but still cause us grief nonetheless. The self conditioned, ingrained negative responses that we all have, to certain outside stimuli.
Instead of changing the pattern of "React, think, suffer" into
"Think, react, minimise negative consequences", we somehow train ourselves into deciding the drawbacks of the original pattern are worth suffering, because we are comfortable with the familiarity. The predictability. There's one example of "Intentional suffering for a predictable life"
It's lazy, it's constrictive, and it's unhealthy. How many times do you hear people say stuff like "I can't help reacting like that, it's how I was brought up"?
When what they really mean is
"This is the response I learned first, when I was a child, and I can't be arsed to learn a more appropriate way to react".
If it's that effortless to replay a "suffering" pattern, then it's equally easy to create a "Stop suffering" pattern, and play that instead.
We will never know why suffering is such a huge dynamic in our cognitive processes, but we can learn to recognise the patterns we use to inflict it upon ourselves, and each other. And. . . . . I don't know, perhaps adjust our own behaviour patterns in order to minimise the damage we do?
"We will destroy those Enemies, but we must first know the Enemy. And the Enemies are the Devils, that hide in our minds, and make us less than happy"
Or we could just sit around and whine about how crap our lives are, whilst blaming every other single factor we can for our woes. Then we can continue to avoid addressing the root cause of most of them. Our own stupidity and reluctance to shoulder the least bit of responsibility for anything we do.
We project our faults onto the nearest unsuspecting scapegoat, and thereby avoid having to actually accept the consequences of our behaviour.
"Well, if I'm too apathetic to stop suffering, I'm going to make sure I'm not alone".
So instead of asking an irrelevant question like "Why is there suffering in the World"? We should be asking "How can I learn to adjust my behaviour, so that it's consequences have positive results instead of negative ones"?
That's a question with many workable and effective answers. Ones that you can implement and see quick results with. This starts new positive cycles or patterns that replace the negative ones.
The cessation of suffering will positively reinforce the new patterns. And patterns are nothing if not viral in nature. They infect the patterns of others around us.
It's like upgrading your AV Software. Introduce a piece of corrective code, instead of the malware that we keep around in our personal runtime environments. Corrective code that replaces and neutralises the malware.
Anything else makes as much sense as having your new Windows 7 Laptop downgrading itself with automatic updates, so you end up with good old XP, or even worse, Vista!