What if, instead of there being 2 rulers, there were supposed to be 3? Here's my theory: Starswirl was supposed to be the third ruler. However, he refused to understand the magic of friendship, and thus, he was not able to turn into an alicorn. He got kinda frustrated, and ended up traveling away. Combine this with the Nightmare Moon incident, and Celestia has been doing the work of 3 alicorns for 1000 years. Celestia needs a break. She is grooming Cadence and Twilight be the new princesses. However, Celestia also wants the rulers to have a stronger bond than before. So, she has had the 2 current new princesses grow up with each other. So, ideally, the third ruler should have a bond with princesses. And who has a large bond with both ponies? Shining Armor. Him and Twilight are destined to do what Celestia and Luna have done.