When I was in first or second grade, a girl who was not the most popular (her last name was the source of much ribbing) asked me to her birthday party. I was not the most popular myself, so I totally would've gone, but she had only given a day's notice so I couldn't.
After the party day, she asked me where I'd been and I told her I just wasn't able to go, to which she replied, "Nobody came."
This has haunted me ever since; it was one of those moments when you feel you understand something new about the human condition (what it is to be rejected), something you won't have to learn again.
Over the years, we went to all the same schools and the same church; we eventually graduated together. She was an optimistic and unfailingly nice person with similarly kind parents.
We reunited, superficially, on Facebook. Over the past year, I noticed she'd had some health issues, but was still surprised to read she died last week. Her last comment on Facebook was something cheerful on someone's picture of their kid.
I really wish I would've been able to go to her birthday party that time.