Much time and energy has been expanding on the question of suffering. Because of this I will not try to explain why humans suffer. I will however, try to give a little of the teenager's perspective. In class we have been reading the book Night, a powerful book about the Holocaust. In the course of our discussion, we began to talk about human suffering, and how, if there is a god, how can he/she/it allow us to suffer. From what we read in Night, the Jewish perspective the author talks about is that the concentration camps were a way from God to punish the Jews for their sins. Personally I, and many of my classmates, find this explanation far from convincing and even a bit repulsive. One of my classmates thought that if this is how God deals with humans, why should we worship God at all? This is the same argument that Wiesel makes in Night.
Personally, I can't explain God's relation to suffering. But something I have noticed, at least with high school students, is that suffering is often a contest. When one person talks about all the homework they have, another person will chime in with "Well I had that much homework AND a sport." Instead of trying to talk about what is going on in their lives, people will try to downplay what is going on in other people's lives when comparing it to their own. They want to be the best, even when that means suffering more.
Everyone suffers. Some people suffer more than others, but that does devalue your own suffering. When we get into a pissing contest over who has more problems in their life, we often ignore the fact that every one's problems are still problems. So when you listen to someone elses' problems, try to remember that, even if what they are going through isn't as bad as what you're experiencing, it is still an issue in their life. So instead of pointing out how your life is worse, just shut up and listen. You might learn something. And at the least, you'll make them better.
For a good look at suffering, the Jesuit Guide to Almost Everything.