What's the saddest thing that's ever happened in life?

Your life? How about my life? The saddest thing that I've ever experienced would be despair. I've been to territory that's worse than depression. One of the saddest moments of my life was when I seen someone that I was supposed to, or meant, to meet with but the last time I seen them was literally the end, and I realized that I'd never see the person or know about the person ever again; I could've made a friend, but didn't get any chance, or opportunity. I can't possibly describe how horrible it felt! Life also takes it's place as the saddest thing that's ever happened. Also, there's a piano song that I once heard that I will most likely never hear again!