On Cutie Marks and Magical Theory, By A. Howitzer

Hello! This is the part where I kill you explain the break-through in Cutie Marks and Magical Theory mentioned in the author’s notes of Chapter 21 of My Little Halo, which I puzzled out while writing said chapter and several before it. Be warned though: This is entirely headcanon and, as solid as it seems, it is subject to change as I find the need, and does not necessarily apply to other works or the show itself. In fact, it probably doesn't.
Also, it contains SPOILERS for Chapter 21 of My Little Halo. Do yourself a favor and read that before continuing; if it isn’t up yet, it will be soon.

Ahem.

There seems to be two different ways to get a mark: Reward and Breakthrough. Most ponies use the Reward method; you just figure out your talent and the thing appears. That’s how AJ, Rarity, Pinkie, and Fluttershy all got theirs; a nice peaceful present for a lesson learned or a job well done.
Breakthroughs on the other hoof, like Rainbow Dash’s, Twilight’s, and now Scootaloo’s, are a whole ‘nother beast. It’s basically one’s innate magical potential allowing one’s special talent to burst out and slap the world in the face with itself. Breakthroughs are flashy, violent, and usually come with a price tag labeled “property damage.” They are also heavy indicators of ponies with a lot of magic power.
And speaking of magic, I think I’ve pinned down my personal theory on what exactly powers it: emotions.
That’s right. Sappy as it sounds, emotions are magic.
But it makes sense, doesn’t it? I mean, magically weak ponies like Trixie and Fluttershy tend to be more cowardly or submissive, and their mood swings are relatively small and dispersed, while arcane powerhouses like Twilight or Pinkie Pie are emotionally charged and can exhibit extreme shifts in disposition over short periods of time. ("Hiii girrrls!")
Artifacts like the Alicorn Amulet would thus of course mess with the user’s emotions because that’s how they grant their power: supercharging a certain range of emotions for you to draw energy off of. The Crystal Heart is so monumentally powerful because it draws in metaphysical (read: emotional/magical) power from an entire city’s worth of ponies to either seed across the land, encouraging the production of similar feelings in its inhabitants, or store in case of emergency, at which point it releases it all in a literal love-bomb.
This also explains why Changelings would feed on love. It’s less physical food and more recharging their batteries, and they only prefer love because it has the most energy. They can “eat” any emotion, but love can be broken down into any lesser feeling in large quantities while taking equally large amounts of weak feels –like sorrow or guilt– to make from scratch. And as I’ve already said, stronger emotions make for stronger magic.
Of course, all of this leads me to conclude that if one burns through magic faster than it can be produced by basic living (like, say, when one is fighting for one’s life and the lives of one’s friends), one also loses the feelings of ones emotions. Particularly large exertions often leave one feeling lethargic, deadened, and in extreme cases lightheaded to the point of unconsciousness. This is often mistaken for, and may in fact be related to post-traumatic shock.
Conversely, this makes bottling up one's feelings an excellent way to build up power over long periods of time, at the risk of increasingly-dangerous instability.