The rule of seven?

[ManaMinori 4,145]

I feel like people feel like there has to be a 7th element of harmony somewhere because good things like this usually come in sevens. 7 chaos emeralds, 7 dragon balls, 7 Millennium Items.... 7 is symbolically looked at as a complete number, while 6 is an incomplete number (such as the 'mark of the best being three 6's, while heavenly mentions are often in 7's) So why does the Tree of Harmony bear 6 elements?

[Goat-kun 2,528]

Five Paladins of Voltron.

No, seriously, magical girls and such usually go with more traditional elements (earth, water, fire, air/ wood, fire, earth, metal, water).

[Ganondorf8 11,367]

My guess is that since Sunset and Starlight had not been created--from our world's perspective--when the concept of the Elements of Harmony came about, only six Elements were created. The idea of there being a seventh didn't likely register with the writers and since this was the official canon, when the Pillars of Equestria were introduced, they had to be six members in total who would possess an Element that would eventually be used to forge the seed that created the tree.

[Flutterstep 47,104]

I always thought Spike would be the 7th main character at the start of the show, but that never really happened. In a way I can see why there isn't any more. I personally think 6 main characters is too much, let alone 7, which I feel only a main character could possess an element to ensure it's not presented as less important.

[Thrond 3,263]

Because seven is a perfect number of main characters, and Spike being a baby dragon doesn't quite fit alongside the pony girls. 

[FirePuppy 735]

The Elements of Harmony is SO Seasons 1-4 (First Era). This is the Second Era we're in now (Seasons 5 and up).

[Steve Piranha 29,432]

On 5/23/2018 at 6:50 PM, Nightmare Muffin said:

I feel like people feel like there has to be a 7th element of harmony somewhere because good things like this usually come in sevens. 7 chaos emeralds, 7 dragon balls, 7 Millennium Items.... 7 is symbolically looked at as a complete number, while 6 is an incomplete number (such as the 'mark of the best being three 6's, while heavenly mentions are often in 7's) So why does the Tree of Harmony bear 6 elements?

I think it's more because 7 is considered a lucky number in many cultures. Still, I can see why a seventh Element of Harmony was speculated. It's a well known cliche of a group of superhero friends being joined by at least an extra member later on. The trope isn't called The Sixt Ranger for nothing