Well, I'm a fanfic writer at heart, so i'm used to needing a justification to get the characters together, at the very least to characterize them to the point where kissing and heavy petting makes sense.
But yeah, when its fanart, you can pretty much make the characters do anything, cus there could always be some sort of context where it makes sense, even if that context is "I'm bored, wanna kiss?" "uhm, sure AJ, why not?"
Ah, I can see the difference in motivation then. Fanfic -is- context. Fanart can be removed from it. In my mind, though, it's not too hard to find a way to push... oh, just about anyone together! The power of allure, combined with suspension of disbelief. If you want it bad enough, it'll find a way to justify itself X)
Oh yes, my OTP is Derpy/The Mayor, and there's like ONE scene that justifies this, in lesson zero where they fight over smartypants and they have this moment where they stare at each others intently in a way that'll totally lead to some hot hatekissing, then they pause and looks around like "oops, you wheren't supposed to see this" [link] it is the most adoreable thing ever! That is ALL the justification I will ever need.