Why is always the big question. Why him/her? What did they do to deserve that? Or in the extreme, why them and not me?
It's easier to answer if the deceased is old or been sick a long time. Especially tough to answer when it's one of those "shit happens" things like a tree falling or getting hit by lightning.
Quite often it ends up the only explanation is, shit happens or God's will. You have to figure out what works for you.