1. He's (more) like the rest of us.
At first glance, this might seem a little, strange. I'm sure you're thinking "Did this clown (pun intended) really just suggest that the most infamous mass murdering psychopath in American pop culture is more relatable than the most famous (yes, I said it) superhero in American pop culture?". Well, yes, I just did.
First, if you were to ever find yourself at your lowest point, which would most likely be your next step? A) Undertake a decade or so of the most intense, world-class mental and physical training that money can buy to become a masked vigilante? Or B) think to yourself "Screw it, I'm robbing a bank!". Most of us, probably all of us, would probably choose option B. But that's why there is only one Bruce Wayne, isn't there?
Secondly, The Joker shares far more similar emotions to the reader than Batman does. The Joker can be happy, the Joker can be angry, he can be sad, he can be laughing, he can be screaming. He even has a 'steady' girlfriend. Bruce Wayne spends half of his time brooding, and how often does the regular person brood?
The fact of the matter is, the Joker is far more like the rest of us than Batman is. This notion is highlighted by two of the most famous Batman adaptations to date, with the "One Bad Day" premise in Alan Moore's seminal "The Killing Joke", and the "They're only as good as the world allows them to be" scene in Christopher Nolan's "The Dark Knight". The Joker truly believes that "all it takes is one bad day" for people to prove that "when the chips are down, these 'civilized' people, they'll eat each other". Sometimes, I can't help but agree.
2. Badass villains are more interesting than badass heroes.
This is one of the most cliched adages in comics, however it remains true to this day: an interesting villain is more appealing than an interesting superhero, especially when you're talking about perhaps the most interesting comic book character of all time. Case in point: look at the two biggest comic book movies of all-time: "The Avengers" and "The Dark Knight". One of the major criticisms of The Avengers was the weak villain presence (when Loki is your best villain you have a serious issue, but that's another article of another day), while the crowning jewel of "The Dark Knight" movie, and overall franchise, is Heath Ledger's portrayal of the Joker. Even in Tim Burton's "Batman", it was the Joker that received top billing. For the role of the Joker, Burton cast the legendary, Academy Award winning Jack Nicholson, but for Batman, he cast a comedic actor best know for his roles in movies such as "Mr.Mom" and "Beetlejuice."
What makes the Joker so endlessly appealing is what's underneath his bleached skin and purple three-piece; his motivations, his desires. Does he want to watch the world burn, or does he just want to mess with Batman? Is he a product of nature or nurture? Is he a result of the circumstances around him or was he born this way? Is he trying to prove a point, or is it all fun and games? Where does he buy his suits? (Kidding, but not really). What makes characters appealing, whether it be in comic books or otherwise, are layers of defining traits. Put it this way, if Batman is the core of the earth, the Joker is the crust. The crust has far more layers and different aspects than the core. While the core is the key to the planet, it is the crust that protects it. With Batman, his motivations are clear; he wants to honor his parents death, he wants to save his city, he wants to do the right thing. While commendable, the desires and characteristics of Batman's soul are very consistent, very straight-forward. As opposed to the Joker, who despite his malevolence, can sometimes still make you question if he actually is the only sane one in Gotham.
3. Every light side needs a dark side.
What does Batman have in common with Star Wars, The Lord of the Rings, and Lost, other than being some of the most successful Hollywood properties of all time? One of their major, if not defining themes are light versus dark, good versus evil, and how the two are opposite sides of the same coin; the idea is that one cannot exist without the other. Throughout their history, the Joker has made sure that Batman knows how true that really is. For example, in The Dark Knight, the interrogation scene highlights this topic, or in finale of "The Killing Joke", where the killing joke is about two lunatics in an insane asylum, one just a little crazier than the other.
This has been portrayed in video gaming as well. In the [Batman: Arkham Series](tag:2586475) video game Arkham City, there is a dream sequence involving the Joker that's hints towards his possible romantic feelings towards Batman, and the purpose he gives his life. Similarly, in Arkham Knight, Bruce Wayne's biggest fears manifest themselves in the form of the Joker. The sheer duality of these two is what makes Batman versus the Joker one of, if not the, most famous relationships in American pop culture.
4. He is literally powerless, except for his willpower.
You have got to hand it to the Joker, the dude is relentless. One of Batman's most endearing qualities is his determination and will, however, most people fail to realize that it might be the Joker who is stronger in both of those categories. Bruce Wayne is literally one of the richest men in the world, with some of the best connections and technology that money can buy. He has the most advanced technology and gadgets known to man. He has multiple vehicles for all sorts of terrain. He has a butler, multiple sidekicks, and (occasionally) an entire police force by his side, helping his cause. He even has other superheroes pitching him from time to time. The Joker on the other hand, aside from Harley Quinn, couldn't be any more alone. The Joker is a true renegade. His most dangerous quality is his sheer will and desire for destruction. Think about how many times he has lost to Batman and been thrown in Arkham Asylum, only to break out a week later and do it all over again. That, in some sick and twisted way, is sort of admirable. After all, look what he can do to a city with a few drums of gas and a couple of bullets.
5. The Batman needs The Joker
While Batman is the hero, he simply would not be the cultural icon he is today without his rogues gallery, especially, the Joker. The most successful versions of Batman, whether it be in the comics, on film, or on television, involve the Joker. The best Batman lines ever written have been in conversation with the Joker. He brings out both the best, and the worst, of the Batman. Simply put, without the Joker, Batman's status as a cultural icon would be no where near where it is today.