“I’ve been solving YOUR little problem for five years! Maybe, for just ONCE in your life, you could solve one of mine!”  
“P-Problem? I’m…I’m just a problem?”
 
When you need someone to blame, there’s seldom a better target than yourself.
 
So, I have this headcanon that Rarity and Sweetie’s parents are super-neglectful and spend all their time traveling. About as soon as Rarity moved out, they dumped Sweetie on her and headed off into the world. So, naturally, they don’t exactly see eye-to-eye on a lot of things, particularly on each other. It does explain a lot of things, especially how Sweetie supposedly lives with them but is always seen at Rarity’s, and the relationship between the two. Rarity tends to view Sweetie as a chore/annoyance (pre-“Sisterhooves Social” anyway), while Sweetie basically worships the ground Rarity walks on… Probably because Rarity is the only member of their family who gives half a shit about her.
 
Context is a beautiful thing: http://rarisweetichief.tumblr.com/post/107219482326/sweetheart-t-thats-not-what-i-meant-at-all