A friend who has no interest in comics recently asked if I'm a Marvel or a DC person. It's a simple answer with a more complex background - I'm a DC boy but now read almost only Marvel. (At least as far as mainstream, ie superhero, comics.)
Dave Barry somewhere talks about how sports fans imprint on a specific team when they're young and stay fans for life. Possibly something similar happens for comics fans. As a kid I read DC and that was pretty much it. Oh I did Marvel from time to time - often enough that I knew the main characters without all the history - and as a constantly restless reader I dipped into Charlton, Dell, Archie and others that showed on spinner racks at that time, including Warren books years before I really should have tackled them.
Why DC? I can't say. Maybe it was goofier, maybe more optimistic, maybe more to my taste, or most likely it was just the first I read. The brother of a friend liked DC better because the stories were usually finished in a single issue, unlike Marvel which had stories that stretched through issues and issues. Today of course they're practically the same in this respect. In retrospect it didn't hurt that DC had a history going back to the 30s which it often reprinted while Marvel seemed to be in an eternal now. Even now I'm surprised at how much older material turned up in DC during the 70s.
2004's JLA/Avengers could have been just another enormous crossover to excite the fans - in many ways that's really all it is. But somewhere along the line writer Kurt Busiek decided it could also be an exploration of the ideology of the two companies. Nothing particularly heavy - this isn't Althusser or Gramsci. The Flash is the first through the separation of the two universes (its own historical reference) and his reports are what DC characters think of the Marvel universe. Different people (mutants) not just persecuted but physically abused. The world smaller and darker. Murderous psychopaths considered heroes (The Punisher). Supervillains running entire countries. The Marvel characters see something different in the DC world. It's shiny and bright. There are museums devoted to dead heroes. Ordinary people ask for autographs. It's Captain America of course who gets to utter the f-word - they're fascists.
Does this explain any of my interests? Not really but it offers hints. Marvel has always been considered more realistic than DC despite that being a pretty subtle distinction for superhero comics. After all Marvel had Dr Doom shooting the entire Baxter Building into space, a Spider-Buggy (seriously, look it up) and Skrulls turned into cows (that hilariously was decades later referenced as one motivation for the Secret Invasion). By the 70s DC was printing Batman stories as psychologically insightful as Simenon and the still-controversial O'Neill/Adams Green Lantern/Green Arrow stories that are about the only time superhero comics successfully grappled with real-world problems.
So why today do I read almost no DC but large chunks of Marvel? Basically it's the New 52 which should have opened the creative floodgates but instead shut them tighter (you can read some of my reactions from when it launched) while Marvel has experimented and put more faith in creators. Before the New 52 DC was publishing more than just superheroes with a Western title, fantasy, crime, SF and some unique ones. Now that's all gone. Today it's Marvel putting out titles featuring a Muslim superhero or a black/Hispanic one, who's taking chances on unusual ideas like Hawkeye, Superior Foes of Spider-Man and The Unbeatable Squirrel-Girl, who have more women characters in more prominent roles, who are simply producing better stories.
Looking back it's pretty clear that the New 52 was driven more by marketing than creative concerns, one reason several artists and writers have gone on record about the difficulty getting work done. For me the titles mostly became too dull and too much the same. Even the war and Western books were turned into superhero titles and Vertigo was partially dismantled around this time. I don't care much about continuity but DC titles got to the point where it became an issue.
But nothing in a product-hungry field like comics lasts - there always has to be something new and as I'm writing this both companies are on the verge of big changes. DC is backing away from some of the New 52 changes and promising more creator-driven titles but we'll see. Marvel is folding its regular and Ultimate lines together and been tight-lipped about the result but right now have a more reliable track record.