Why should we mourn this person? Don't you people know there are all sorts of other sad, depressing, shitty, soul-crushing things going on in this world? Aren't those more important?To which I say, yes, there are all sorts of other sad, depressing, shitty, soul-crushing things going on in the world. The Universe is a sad, depressing, shitty, soul-crushing place. Pretty much all of the time.
And it's because The Universe is a sad, depressing, shitty, soul-crushing place we need people like Gene Wilder. And Prince, David Bowie, Robin Williams, Ayrton Senna, Dale Earnhardt, Justin Wilson, Jim Henson, Leonard Nimoy, Muhammad Ali, and many, many other people that have left us and whom we mourn. (Apologies if I've forgotten your favorite celebrity/sportsperson/politician/whatever. I can't please everyone.)
For a few hours of our lives, they give us some relief from what a horrible place The Universe is and what it's doing to us. No, I didn't know Gene Wilder personally; very few people on this planet did. But I do know what he did, how he made me feel, how he made me forget about the sad, depressing, shitty, soul-crushing planet on which I live, even if only for a time.
It's not that I don't care about the horrible things that The Universe throws at us; I do. Personally, I think that you'd have to be a pretty terrible person if you don't. There are plenty of people out there that don't, and we call those people assholes.
We can either focus on what a sad, depressing, shitty, soul-crushing place this world is, or we can remember that there are people in our lives—it doesn't matter if we actually know them or not—that make the world a better place. Even if it's only briefly.
Go out and protest injustice. Contribute to charities that bring food, water, and medicine to people who don't have easy access to them. Vote for whatever politician you think will make things better. Make stuff. Do stuff. Or just focus on making your life and your family members' lives better. Whatever you want, that's fine.
We mourn Gene Wilder (and others) because he brought a ray of light into this big, dark Universe, and now that light is gone.
Thank you for the ray of light, Mr. Wilder. I'm glad I was able to witness it. Rest well.

