Rarity: “Are you accusing me of slacking off? It seems like you’re confusing me with yourself!”
 
Applejack: “Ah don’t think so. That’d be a real tricky thing for me to do; comparin’ mahself to you.”
 
Rarity: “You’re right, the difference between us is like heaven and hell.”
 
Applejack: “An’ it ain’t hard guessin’ which one you might stand for.”

What the fuck did you just fucking say about me, you little bitch? I’ll have you know I graduated top of my class in the Navy Seals, and I’ve been involved in numerous secret raids on Al-Quaeda, and I have over 300 confirmed kills. I am trained in gorilla warfare and I’m the top sniper in the entire US armed forces. You are nothing to me but just another target. I will wipe you the fuck out with precision the likes of which has never been seen before on this Earth, mark my fucking words. You think you can get away with saying that shit to me over the Internet? Think again, fucker. As we speak I am contacting my secret network of spies across the USA and your IP is being traced right now so you better prepare for the storm, maggot. The storm that wipes out the pathetic little thing you call your life. You’re fucking dead, kid. I can be anywhere, anytime, and I can kill you in over seven hundred ways, and that’s just with my bare hands. Not only am I extensively trained in unarmed combat, but I have access to the entire arsenal of the United States Marine Corps and I will use it to its full extent to wipe your miserable ass off the face of the continent, you little shit. If only you could have known what unholy retribution your little “clever” comment was about to bring down upon you, maybe you would have held your fucking tongue. But you couldn’t, you didn’t, and now you’re paying the price, you goddamn idiot. I will shit fury all over you and you will drown in it. You’re fucking dead, kiddo.

@Shy-Brony  
Like I said, night and day on the surface, but shockingly similar at the core.

@Scrounge  
Forget similar interests, they’re two sides of the same coin!

@Shy-Brony  
Both have a business which they call both their home and their life’s work. Both have a younger sister whom they love dearly, but don’t always get along with. And both have a deep devotion to the rest of the Mane Six.

@Scrounge  
How do you mean? About their priorities, that is.

@Dragonmage  
Maybe AJ’s argued with everypony at least once, but Rarity and Rainbow Dash are way in front of the pack in terms of being her most frequent sparring partners, and I believe Rarity’s leading by a nose at this point.
 
But anyhow, never said that was the only reason.
 
Rarity and Applejack have a nice dynamic in that they seem completely antithetical on the surface, but underneath, they’re more alike than either would readily admit. For starters, both have similar priorities.

@Background Pony #780A  
@Scrounge  
You do know that AJ’s fought with all the mane 6 at this point, right? She’s the only one in the group to pull it off too. So “because they fight” isn’t really going to cut it at this stage.

@Background Pony #780A
 
Rarity: “You know, Applejack, you and I are one of the classic love stories.”
 
Applejack: “Aw, shucks, ya mean it?”
 
Rarity: “Certainly. Haven’t you ever heard of ‘Beauty and the Beast’?”
 
Applejack: “…Sure you ain’t thinkin’ ‘bout ‘The Farmer an’ the Viper’?”
 
Rarity: “Touche, dear, touche.”

@Scrounge
 
Agreed.

@Background Pony #780A  
It’s also what makes them entertaining, either as platonic friends or a romantic couple.

@Dragonmage
 
@Scrounge
 
Yup. Teasing and squabbling, but also always making up and unable to stay away from each other? It’s like kids crushing on each other, classic romantic tension.

@Dragonmage  
Oh come on, you know they only tease each other because that’s how they flirt. :P

@sumbozo  
Nah, AJ should be with someone she gets along with better, like Twilight. ;)

Oh, just kiss already!