Only a year after Superman entered the world, the comic book readers of 1939 found themselves faced with another new superhero: the Batman. This hero had a compelling personality, a sympathetic backstory, and a thirst for justice that quickly won over fans. These traits resonate 75 years later, giving the caped crusader a longevity–and an influence–as powerful as the boy in blue's.

DC Entertainment has declared July 23 "Batman Day", but what is it about the hero that makes us want to celebrate him? For DC co-publisher Jim Lee, Batman's story withstood the test of time because it has something everyone can relate to and at the end of the day he's a crusader for justice.

"He fulfills a deep wish, which is basically 'wouldn't it be great to have a crusader watch over us and make sure nothing bad happens?' It's a character who has dedicated his life to that mission," Lee said. "He's sympathetic because his parents were murdered, but he took that tragedy and basically willed himself to become the best fighter and detective possible. Out of that came this incredible character with no superpowers, but who can still stand toe to toe with the greatest superheroes in DC mythology."

From the quirky 1960s TV show to Christopher Nolan's film trilogy, Batman's been seen in a number of different incarnations. In comics, creators adapted him to the times, embracing sci-fi in the 1950s and urban decay in the 1970s. According to Dr. William Brooker, author of Batman Unmasked: Analyzing a Cultural Icon and Hunting the Dark Knight: Twenty-First Century Batman, he's a very adaptable icon with strong core aspects that remain the same no matter the version of the character. To Dr. Brooker, the enduring presence of Batman and his qualities in our pop culture says more about the hero than our culture itself.

"Our culture has changed vastly since 1939. The remarkable thing is that Batman has changed, adapted and remained relevant to it. Batman launched before Pearl Harbor. He's now lasted 14 years past 9/11. The first Batman comics were shared around by young men fighting in World War Two. Now Batman comics are downloaded … it's pretty incredible that Batman has stayed with us on that ride for so long," Dr. Brooker said.

Dr. Brooker believes Batman's impact can be seen the most in the superhero genre itself, however, as he was one of the first—and has had a huge influence on those that followed. The "vigilante" subgenre (Think The Punisher or The Question) owe "a debt to the Batman, just as every optimistic, idealistic do-gooder superhero is, on one level, a variant of Superman."

The caped crusader also became the first blockbuster superhero movie franchise, with 1989's dark, epic Batman earning more than $400m at the box office.

But why do we keep coming back? Why do the core elements of his story resonate with us so much? Why remake him, again and again>

"He's vulnerable from the very beginning. From the campiest version down to the grittiest, we see he struggled, and he does struggle with his darkness and he does wrestle with his abilities," said Dr. Andrea Letamendi, a clinical psychologist who speaks about psychology and comic books at various conventions across the country. "It's a universal trait and a reason we're drawn to him. We want to see him succeed and be the best superhero he can be. We're also drawn to him because we see someone who struggles and that's the life experience."

Dr. Letamendi, who also co-hosts a podcast that offers a psychological analysis of Batman: The Animated Series called The Arkham Sessions, believes the human aspect of Batman is especially important to readers. He doesn't have superpowers and the catalyst of his origin, while tragic like many other heroes, isn't super. He doesn't gain super-strength or hide alien powers behind his alter ego, Bruce Wayne. Batman and Bruce are both human, and as a result there's something human about the transformation between the two, says Dr. Letamendi.

Put simply, he's more like us than the others: a person who has seen tragedy and trauma, but still finds the strength to try to make the world a better place. This is how a character become archetypal; it's what'll keep Batman around for another 75 years.

"He existed before I was born, and will exist well after I shuffle off this mortal coil," Lee said. "He's weathered the decades and as long as he stays true to that core mythology and stays updated so he feels contemporary, his appeal will continue to last."