There sure are a lot of time travel shenanigans in the Marvel universe. Besides making it really hard to remember which age we're on (Age of Apocalypse, Age of X, Age of Ultron, or the Summer of George) it really mucks up who has their origins in what timeline.
This is a problem that most typically hounds the X-Men. What with them being so concerned about their survival as a species. One Hank "Beast" McCoy even recently revealed he's time traveled so much that it's left him "attuned" to changes in space-time. Which is a useful talent when someone like Kang the Conqueror starts erasing your friends from existence.
Time travel is not, however, always a uniquely mutant problem. Enter Robert Bruce Banner, host to The Incredible Hulk. Or the Grey Hulk. Or Joe Fixit. Or any number of other looks and personas the Jade Giant has manifested over the years. Take a look at the Marvel Database listing for Bruce Banner's known aliases sometime if you've got a spare afternoon. The guy tends to jump names more often than a Spider-Man villain jumps bail.
In reality, these differences tend to vary by writer and artist. Different creators tend to have different ideas of what the Hulk should be, and with an established history of unstable identities he's a perfect target for creative license. Though more often than not these scribes tend to avoid in-fiction explanations. So exactly why the big brute goes from speaking like a normal person two issues ago, to barely forming complete sentences gets lost in the hand-over.
Enter Mark Waid. As a decades-long writer for both DC and Marvel the dude often knows his comic book history. Knowledge he tends to leverage in stories like the outstanding Kingdom Come, and a stellar, lengthy run on Daredevil. So when Marvel handed him the reins to the Hulk it only took him about 10 issues to address the character's shifting facade.
In Waid's series, Indestructible Hulk, a S.H.I.E.L.D. prisoner and chronological bad guy The Tomorrow Man gives Banner the skinny. Time traveling curs have been destabilizing history for personal gain. Side effects include a Hulk whose very origins change with the tides. Leading to a series of very different Hulks all the live-long day. Banner and the big guy proceed to right any historical wrongs, of course, but not before time was already well out of whack leading to the roster of Hulks we all know and love. Not to mention fear.
Salt and pepper. Peanut butter and chocolate. Simon and Garfunkel. Some things are just meant to be presented in pairs. One such example from Marvel Comics is the truly dynamic duo of Cable and Deapool. Separately, the pair can lead or season a book on their own. Together, though, the pair just tends to "work" in that straight-man-and-goofball combination that's lasted through the ages.
Hence the pair being constantly thrust together in team-up book, after miniseries, after crossover. The most notable of which is certainly the four-year, 50-issue series appropriately titled Cable & Deadpool. Though that might change with the next Deadpool movie. Cable's eventual involvement on the big screen was implied during the after credits stinger from the first film.
But why are these two so inseparable? The real answer is, of course, because it works. Cable is the time travel hardened son of Scott "Cyclops" Summers and Madelyne Pryor (the sometimes evil clone of Jean Grey). In short, he's a real stiff -- having been created by one Rob Liefeld during the height of 80s rough-and-tumble action-man comic book writing.
Then there's Deadpool: a complete yet effectual idiot (also created by Liefeld) Foils work well as friends in fiction, and so the pairing was born. We're talking funny books, however, and sooner or later everything in the realm of comics gets a (sometimes unnecessary) canonical explanation.
Said explainer for these two comes relatively recently in Marvel fiction. 2014's Deadpool vs. X-Force brought the two characters together for the very first time in a story which takes place during the early 90s. During which time Deadpool was less the happy-go-lucky smack-talker modern writers have morphed him into, and more a "hired killer of innocents." A character facet which was not forgotten by writer Duane Swierczynski.
The story sees Wade Wilson sent back by a war profiteer to rewrite history in the latter's favor. To do so he ensures a Confederate takeover of the United States, bodyguards Hitler well into the 60s, and becomes a Nazi minister with a gun that goes back in time to kill people's parents. Cable, and his as-yet-unformed members of X-Force, takes umbrage with that. A tussle ensues, and Cable as well as Deadpool tumble through a time portal together.
As a result, D & C's fates were doomed to intertwine forever. Or at least for 50 issues and a movie.