Personally, I think that Rarity and Spike are not a good pairing... She's too materialistic to even consider treating him the same as any of her pony friends. Seriously... She treats him about the same as she treated the Diamond Dogs. Maybe that's her ploy? Use and abuse him till he gets fed up finally? :/ Honestly, Rarity is my least favorite of the 6...
As for Twilight... Twi and Spike are siblings moreso than anything really. I've an older sister and when we were younger, she basically did all the same kinda things that Twi does to Spike to me... Sibling relationship of an Older Sister obsessed with something generalized and a happy, friendly, helpful baby brother who's been brought up by a sister and maybe the mother most. Neglect and abuse are built into a happy, healthy sibling relationship. :3
Of the other 6, I feel Pinkie would be like a best female friend type character... with AJ a close second. They both care about him more greatly than any of the others...
And finally about Spike... He's a baby dragon, However he acts more like he's 13-14... I can honestly say that, as a younger brother, when I had hit 13, I started a crush on the prettiest of my sister's friends... it never went anywhere but I still did stuff for her and the rest of my sister's friends. It might have been cause I was hormonal, but as it stands, its completely normal. Seems like Spike's just your average teenager in puberty that was raised with an older sister that's a brainiac... Since he helped her so much with her research and stuff, he learned a lot of what she knows, so he's a teenager that's smart, submissive with a hostile side, and loyal to a fault...
The only thing that really needs changing is Rarity and Spike really, but that's a shtick they do so, not likely... :/