We all hate it when our favorite characters get killed off. It may seem that the story can hardly continue without them, but it may be best if it does.
Comics in the 1990s often focused on long story arcs and crossovers, and Superman was no exception to the trend. The writers behind his four monthly series had planned for Superman to finally marry Lois Lane in a 1992 story arc. However, DC Comics decided to put the story on hold to coincide with a similar story on the Superman TV show of the time, Lois and Clark. The writers suddenly had to figure out what to do with an entire year of Superman comics. What started as a joke during the writers' planning sessions led to one of the most famous arcs in comics history: "The Death of Superman."
Over the course of a seven-issue story arc, they pitted Superman against the unstoppable killing machine Doomsday. The two fought halfway across the country until they finally beat each other to death and Superman died in Lois Lane's arms. The story was followed by the "Funeral for a Friend" arc, in which the DC universe mourned Superman's absence, and "Reign of the Supermen," which featured four characters competing to carry on Superman's legacy. However, the character was simply too integral to the DC universe to stay dead. Thanks to a combination of advanced technology and his alien physiology, Superman bounced back about a year after his death.
Major character deaths had been rare in comics beforehand. However, the comics industry realized that high-profile deaths didn't have to be permanent, and could be great for sales. Since the early '90s, half the superheroes worth mentioning have been killed and resurrected. Even characters killed years before "The Death of Superman" have returned in recent years. Death in superhero comics has simply become too reversible, and doesn't carry much weight anymore.
This trend is due in no small part to the challenges of a serialized medium. The most popular heroes have been appearing in comic books non-stop for decades. Readers require major, dramatic events in order to stay interested, but tend to react poorly if the status quo is upset too much. Killing a hero provides a momentous event, and bringing them back reestablishes the comfortable norm. As superhero films shift from standalone blockbusters to chapters in larger stories, similar tactics have been employed in recent movies as well.
More recently, DC and Marvel have taken an interesting approach to reintroducing dead characters. DC introduced a new version of Superman in their company-wide 2011 reboot. The rebooted Superman proved to be less popular than the previous incarnation, and was thus killed off quite definitively. Through a series of events too convoluted to get into here, the pre-reboot Superman reappeared, now accompanied by his wife Lois and their young son, Jon. Similarly, Marvel killed off the main version of Wolverine a few years ago. Rather than resurrecting him, they've chosen to focus on Old Man Logan, a version of Wolverine from an alternate universe in which he is the last hero alive.
This strategy preserves the finality of death, and allows these characters to break free of the status quo. Obviously, this solution will get equally tired as resurrection if overused as much, but it's a step in the right direction.