Definitely not being able to see your loved ones in your last moments.
Explanation:
When you're nearing death and on your death bed, you generally want people you know and love to be there so you can say your last goodbyes and say how much you loved them, and so they can do the same for you. However, if there's nobody there in your last moments, it's just this feeling of loneliness and sadness as you pass on.
My opinion on this answer:
In my opinion, that's one of the things I'm scared of. One of my fears is being alone, and if that ever happened to me I feel like my last few moments of living would be filled with tears.
The laughing man in the book When You Reach Me seems crazy because he practices punches to save Sal from getting run over by a truck. The laughing man in reality was Marcus (the kid who tries to explain to Miranda the concept of time travel) traveled back in time.
