One theory that exists is that which states we are spiritual beings who incarnate as humans time and time again in order to grow and experience certain things (ultimately everything) in order to achieve a more enlightened state. Acts of love, altruism and empathy, as well as enduring hardships are supposedly some of the fastest ways of getting there. The theory states that we volunteer to have a human experience in what's known as a life contract whereby we're tied to a certain body or life for the duration of an incarnation in order to overcome whatever we struggle with most. Whether there's anything to all of this I don't know, but it could explain why life is difficult/cruel. Many religions also preach the virtues of suffering. We do live in a pretty nihilistic society and time period, but I wouldn't necessarily rule out any of the above on principle. I know it's tempting to shit on anything that's unscientific and that people are generally terrified of being seen as naive or airy fairy, but I do sometimes find that I almost need to believe in a greater goal for all the suffering I go through. I'd hate to think it was all for nothing.