Deadpool first debuted in 1991 riding two very lucrative trends in comics at the time. First was the "Dark Age", where young artists were inspired by like creators Alan Moore and Frank Miller to create characters who were more edgy and violent (something that kids at the time gobbled up as "more mature"). Also raking in the dollars was the "collector's boom", where older fans began to buy up and inflate the price for any seemingly "significant" issues under the impression that they would be worth thousands, if not MILLIONS of dollars someday. So when Rob Liefeld, one of the grittiest new talents at Marvel was introducing dozens of new characters in his wing of the X-Men universe, they were moving books at a rate that publishers and distributors could barely keep up with demand.
But with every bubble inevitably comes a crash. By the late 90s and the 2000s the same characters had been selling out at toy stores were barely moving off the shelves and comics themselves were being bought at a fraction of their former heights. Yet while many other Liefeld creations were shelved and lost to time, Deadpool was far scrappier than that. His seeming irrelevance in the modern superhero landscape actually gave his writers unprecedented freedom to tell their own stories in the margins of the Marvel Universe without any meddling from the big wigs in editorial.
By the time the world wide web began collapsing and social media became the predominant form of online media, Deadpool had amassed hundreds of funny and sharable moments from over two decades of free spirited comics. Plus his "tactical ninja" design with all the pouches and weapons made him stand out from the usual spandex menagerie, even though it was overplayed at the time. Deadpool was dealt a bad hand from an economics standpoint, but thanks to tenacity and raw charisma, he's become and indelible part of our culture.
Writer Fabian Nicieza was a longtime Marvel employee who worked his way up from the marketing department to writing and plotting for some of the company's hottest books. One of those writing gigs included working alongside a very (VERY) young Rob Liefeld, whose scratchy linework and exaggerated anatomy was like catnip for the 12-17 year olds who were buying X-books at the time. So when Liefeld designed a sword-wielding mercenary to antagonize his other hit character Cable, Fabian decided to give him a wiseass sense of humor to make him a contrast for the stone-sober Cable. Deadpool was deadly but unserious, a physical threat who delighted in mocking and embarrassing his enemies. Cable made battle plans and talked about "The Mission" while Deadpool MADE JOKES.
To write for Deadpool is to write jokes, and writing jokes is HARD. Eventually, Deadpool began to comment on movies, politics, and other topical references that a character might not necessarily be aware of "in-universe". Writers like Joe Kelly, Gail Simone, and Daniel Way began to make fun of other Marvel titles and even the mechanics of comic books themselves. Deadpool was so "above it all" that he could make fun of being a comic character himself and speak directly to the readers. Obliterating the 4th wall as long as it was good for a laugh. This kind of direct testimonial style drives the same kind of connection that has made Reality TV and YouTube Stars possible. Millenials value sincerity and directness vs the old showbiz tricks of generations passed, so only Deadpool can resonate with them on this intimate of a level.
While Deadpool's origin has gone through many reboots, rewrites, and retcons, the common elements all involve Wade Wilson's sudden diagnosis of Stage 4 cancer and submitting himself to military experimentation in the hopes of surviving. Considering how most writers also give him a background in rural childhood poverty, it's clear to see that Deadpool speaks to the desperation and upheaval that arises from a bad diagnosis. Most American millennials have had negative experiences with the healthcare system, especially the bizarre ways that it's tied to employment. That his origin is tied to a CANADIAN government facility (either Weapon X or Department K, depending on who you ask) is just too appropriate.
Oh, the fact that he's a mercenary means that he has plenty of experience with temporary contract work with no benefits.
Deadpool's manic energy manifests itself in an almost childlike obsession with gadgets. While most millennials will fawn over their new cell phone or showcase their latest fashion acquisitions on social media, the Merc with a Mouth is obsessed with the near limitless varieties of weapons at his disposal. Besides giving his favorite swords and pistols their own pet names, Deadpool's collection also serves the creative team, since there seems to be no challenge so great, no foe so insurmountable that it can't be solved by "Deadpool showing up with a bunch of rocket launchers".
You wouldn't think that the poster boy for crude "anti-PC" comedy was a bleeding heart SJW, most modern Deadpool writers have made it clear that the guy has a heart of gold and is dead set on being your "unproblematic fave".
It makes for some solid moments of comedy, a seemingly remorseless killing machine that cares more about a downtrodden diner waitress than the legion of security guards he just slaughtered. But folks like Gerry Duggan have written stories where Deadpool proves that he knows American Sign Language, has a half-Hispanic daughter, and is canonically pansexual.
There's a lot of positivity in Deadpool books (if you look past the recurring themes of death, mutilation, and torture). While the subjects of his mockery will inevitably shift from writer to writer, there's lots of stories out there where Wade does his best for the downtrodden and oppressed. Kind of ironic that this self-effacing character was born from what was the most overwhelmingly serious and hyper-masculine era of comicbooks.
"Funny" is an elusive target, always moving and changing with the times. So after 2+ decades of adventures it's easy to see how some of Deadpool's bon-mots don't age particularly well. But for the #1 metahuman for millennials, they wouldn't have it any other way.
Often criticized as a "Hot Topic Hero", Deadpool can induce cringes just as often as he induces laughter, but can't the same be said about your facebook timeline or tumblr blog? Deadpool doesn't look to the past, he lives in the NOW and if the NOW means that sometimes you have to throw in a crack about "fidget spinners" that ages worse than last week's caeser salad, then so be it. YOLO. Damn Daniel.
People criticize memes as shallow but what they really are is a social connection. An in-joke that can be shared by absolute strangers across the world. While other heroes are too concerned with their legacy to indulge in such triviality, Deadpool has no legacy to stand on, he's a freak who somehow managed to survive some crazy bullshit and has the shameless sense of humor to prove it.
Just like the rest of us.