How fortunate are we that the show has a roughly even Mane 6 dynamic

[GuillermoGage 246]

Under the control of lesser creators, the show could've started out being a roughly equal ensemble in the theme song and some of the two-parters/The Best Night Ever, 

 

but then ended up as a show like SpongeBob where there were essentially three or so characters where the main protagonist interacts with regularly (In SpongeBob's case, that would be his next-door neighbors, one of which is a co-worker, and boss), with other characters only appearing when they happen to cross paths with him (such as Sandy and Plankton)
In MLPFiM, the other five ponies have roughly an equal chance of interacting with Twilight. It's not like some are particularly more close to her than others. 

FiM easily could've made the show a girl version of a dumb buddy antics show with Rarity being her Patrick(SpongeBob and Patrick are both childish, Twilight and Rarity could be BFFs because they are both unicorns)

AJ, Fluttershy, Pinkie, and Rainbow having various shades of pridefulness combined with straight-man voice of reason, or nuggets or wisdom, like Squidward and Sandy...

...with Rarity being directly involved if not the catalyst of the events that Twi gets involved in on a regular basis for at least half the episodes (just like how Patrick is alongside SpongeBob for many antics, and Patrick never interacts with other characters of his won accord without SpongeBob being involved)

with Princess Celestia or some Ponyville authority character such as the Mayor appearing much more often, possibly moreso than a "Sandy" (Mr. Krabs) and a consistently reoccurring nuisance antagonist (like a young mare, non-filly version of Diamond Tiara and Silver Spoon).